Require fish 3.2.0, update variable preview extraction (PatrickF1#146)

Now that fish 3.2.0 has been out for almost a month, it's time to make it the minimum required version. Thankfully, the only new expectation made by the plugin is around the new `set --show` format. It has been simplified (see fish-shell/fish-shell#6944) such that the search shell variable preview logic, which uses `set --show`, can be greatly simplified. While this is technically backwards incompatible with fish 3.1.2, the only negative consequence for users who haven't upgraded should be that their variable preview won't be as clean and condensed but will still have all the essentials.

Remove problematic char in __fzf_search_current_dir (PatrickF1#142)

There was a zero-width character that came from copy and pasting code from the GitHub web interface. It's invisible but can be seen by running git diff 2c0fb53 2c0fb53^ | vim -

[Search directory] expand variables in current token (PatrickF1#133)

Evolved from PatrickF1#109. Now not only does tilde (~) in the current token get expanded, variables do as well. This is useful for quickly accessing known directory variables, e.g. $XDG_CONFIG_HOME, $TMPDIR, $JAVA_HOME, $fisher_path, $__fish_user_data_dir.

[Search history] preview commands with fish_indent --ansi instead of …

…bat (PatrickF1#135)

Why fish_indent instead of bat? It ships with fish, it looks exactly like how the fish command line highlights the text, and it's significantly faster.

Remove leading underscores from fzf_search_vars_cmd

Since fzf_search_vars_cmd is meant to be exposed and used by the user (for users who customize their key bindings), it should not start with __, which is conventionally used to signal that the entity should be off limits to the user.

Make preview directory command configurable (PatrickF1#120)

To acquiesce to the popularity and diversity of ls alternatives (e.g. https://github.com/ogham/exa, https://github.com/Peltoche/lsd, https://github.com/athityakumar/colorls, tree, all of which have many different options to configure them), I have finally decided to make the command used to preview directories configurable. Users can place their desired command in fzf_preview_dir_cmd and fzf.fish will use it.

Related to PatrickF1#117 and PatrickF1#101

Expand tilde from current token while searching files (PatrickF1#109)

Before, vim ~/<ctrl-f> did not work. Users had to manually expand ~ (e.g. vim /Users/aaa/<ctrl-f> worked fine). This is because the test built-in and fd doesn't expand ~ automatically, so we have to expand it ourselves.
Fixes PatrickF1#88

Prevent arguments from being interpreted as options (PatrickF1#80)

Prevent errors that can happen if the current token, selected path, or selected command, beings with a `-`. 
Examples where fzf.fish would error:
- the cursor is over the token --something when the user executes search files
- the cursor is over -z when the user executes search variables
- the selected changed path from git status is -folder
- the selected command is --function

The solution is to delineate the end of options with --, which is a POSIX specification defined in https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02 (guideline 10), which is respected by Fish builtins. After --, the command will always interpret the arguments as positional arguments.

Allow quick open with pre-populated query and non-directories (Patric…

…kF1#76)

Make the quick cd behavior trigger even if the user was starting to type a token and even if the path is not a directory. Before, it would only prepend ./ if the command line was empty and the selected path is a directory.

Now, if the user was in the middle of inputting the first token and only one path is selected, then prepend ./ to the selected path so that
 - if the path is an executable, the user can hit Enter one more time to immediately execute it
 - if the path is a directory, the user can hit Enter one more time to immediately cd into it

Limit file search to directory under cursor if it has a trailing / (P…

…atrickF1#75)

This new behavior allows the user to control the directory in which the search file feature initiates its search. If the current token is a directory and it has a trailing slash (e.g. functions/<CURSOR>), then we assume the user wants to search for files in that directory.
This can be useful when in a directory containing large subdirectories (e.g. on macOS, the home directory contains ~/Library which is very large). Moreover, it also allows searching a parent or sibling directory by using ../ (e.g. ../Downloads/<CURSOR>. 
What's great is that this new behavior is that it complements tab completion. When a user wants to access a file in a subdirectory, they may start typing the name of the directory and hit TAB. When tab completion writes out the directory's path, it will already append a trailing slash for the user, allowing the user to quickly execute the search files feature and search within that directory.