Simplify selector combinators (#20089)

This PR is a follow-up of #20088 to further improve selectors, in
particular the `combinator`.

This PR explicitly types the `combinator` as:
```ts
type Combinator =
  | ' ' // Descendant combinator
  | '>' // Child combinator
  | '+' // Next-sibling combinator
  | '~' // Subsequent-sibling combinator
```

This allows us to explicitly test for this pattern in various places,
without us having to call `.trim()` first to know what the actual
combinator was.

In the selector parser itself, we already did a `.trim()` to know
whether we were dealing with a descendant combinator or not. With this
PR, we further ensure that there is no whitespace involved aroudn these
combinators.

This introduces a small problem because we need to be able to re-print a
selector's AST. So if we don't track whitespace, we have to re-introduce
it. But there are situations where we don't want it at all (during
canonicalization).

To solve this, we introduced a `minify = false` option in the
`SelectorParser.toCss`. If it's false (the default), then we introduce
whitespace, otherwise we remove all whitespace.

## Test plan

1. All existing tests pass
2. Manual cleanup/trimming of descendants is no longer necessary

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Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>