Provide parser error when :global appears without selector in CSS

[CMessinides]

Fixes #4930

Prevent a compiler crash when :global appears by itself in a CSS selector by adding an extra check when walking the style AST. If a node meets the following conditions:

• type is 'PseudoClassSelector',
• name is 'global', and
• children is null,

then the parser reports an error: :global() must contain a selector, cannot be used on its own.

Long-time user, first-time contributor -- please forgive any naive errors in putting together this PR. In particular, I wasn't quite sure of the right way to report the location of the error. In the file I touched, src/compiler/parse/read/style.ts, one of the existing checks uses the node's loc.start.offset property, while the other uses the start property. The former seemed to make the most sense with the test case I wrote, so I went with that -- but happy to correct if I was wrong.

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Tests

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[tanhauhau]

The former seemed to make the most sense with the test case I wrote,

yea the latter one does not make any sense to me too.. maybe we should fix it together? 🙈

[tanhauhau]

but besides that, overall looks good to me 👍

[Conduitry]

I'd like to avoid adding more compiler error codes whenever possible, because these become a weird undocumented kinda-public part of our API. Is there an existing one that makes more sense here? Maybe just the general one that we use when the underlying CSS-parsing library throws an exception?

[CMessinides]

@Conduitry sorry for the radio silence on this -- code just wasn't front of mind last week. Thanks for making that change and happy to see it merged in.