Issue: #11

The schema are used used by `layout.margins`, so I move them out of
primitives.ts

Users could specify `layout.sections.order` if they want to prioritize
some sections to the front of the final PDF output.

Issue: #12

When users are intented to write a customized date that cannot be parsed
by `Date.parse()`, then we should respect users' intention.

- buymeacoffee links
- compiler/schema demo

Originally the build output:

```
$ pnpm core build:prod

> @yamlresume/yamlresume@0.5.0 core
/Users/hanyu/work/yamlresume/yamlresume
> pnpm --filter @yamlresume/core build\:prod

> @yamlresume/core@0.5.0 build:prod
/Users/hanyu/work/yamlresume/yamlresume/packages/core
> tsup --dts --minify --sourcemap --tsconfig tsconfig.prod.json

CLI Building entry: src/index.ts
CLI Using tsconfig: tsconfig.prod.json
CLI tsup v8.4.0
CLI Using tsup config:
/Users/hanyu/work/yamlresume/yamlresume/packages/core/tsup.config.ts
CLI Target: esnext
CLI Cleaning output folder
ESM Build start
DTS Build start
ESM dist/magic-string.es-PGO5ZCT3.js     21.44 KB
ESM dist/index.js                        99.37 KB
ESM dist/magic-string.es-PGO5ZCT3.js.map 95.28 KB
ESM dist/index.js.map                    334.78 KB
```

So it seems that rollup/tree-shaking will generate `magic-string-xxx`
thing in the build output.

A possible reason:

```
The issue was that production code was importing nullifySchema from
utils.ts, which also contained test utilities that imported vitest. This
caused vitest (and its dependency magic-string) to be included in the
production bundle. By separating the production schema utilities into
their own file, we've eliminated this dependency leak.
```

This refactor get rid of magic-string thing in the final bundle output.