Improve whitespace handling during canonicalization (#19986)

This PR fixes a printing bug during canonicalization where it converts:
```
[&:has(~_*_*:checked)]:text-green-500
```

into:
```
[&:has(~**:checked)]:text-green-500
```

This is because the `_` was marked as insignificant and therefore
removed. This PR fixes that and maintains the whitespace (`_`)
characters when needed.

Additionally, in the comments of the linked issue somebody mentioned
that:
```
w-[calc(100%_-_--spacing(60))]
```

was turned into:
```
w-[calc(100%---spacing(60))]
```

...and while that's still correct and parseable, it's not the prettiest.

This PR will still get rid of the whitespace, but introduce wrapping
parens `(…)` instead, in case readability is not ideal.

In this case, we will turn it into:
```diff
- w-[calc(100%_-_--spacing(60))]
- w-[calc(100%---spacing(60))]
+ w-[calc(100%-(--spacing(60)))]
```

Of course there are some cases where we don't need to introduce `(…)`
unnecessarily:

- `shadow-[inset_0px_1px_--theme(--color-white/15%)]` would not be
turned into `shadow-[inset_0px_1px_(--theme(--color-white/15%))]`
because no readability is gained when it's part of a normal space
separated list
- `m-[--spacing(12.34)]` would not be turned into
`m-[(--spacing(12.34))]` because there is nothing else it can conflict
with
- `m-[calc(--spacing(12.34)*2)]` would not be turned into
`m-[calc((--spacing(12.34))*2)]` because it's the first argument and
doesn't conflict with the `*`
- `m-[min(100%,--spacing(12.34))]` would not be turned into
`m-[min(100%,(--spacing(12.34)))]` because a `,` doesn't cause
readability issues


Fixes:
https://github.com/tailwindlabs/tailwindcss-intellisense/issues/1544

## Test plan

1. Added new tests to ensure the bug is fixed
2. Added new tests to ensure readability is improved (and not degraged)
when whitespace was used to improve readability