Update dependency again.

It looks like I updated golang.org/x/net just a little too soon.
Now there's a vulnerability that could conceivably affect cascadia
users. So we're updating again.

Update Go dependencies

Signed-off-by: Jauder Ho <jauderho@users.noreply.github.com>

Fix index out of bounds in parser.

When parsing an invalid selector like "[a=a i", the parser would panic
with index out of bounds.

Add ignore case feature for CSS attribute selectors

Add one more test for unicode escapes.

Add specificity support (#39)

add support for specificity for simple selectors

Add go.mod file for vgo.