Preprocess self closing style

[lukasmoellerch]

Implements #5080

• Extends the regex that is used to identify script / style tags during preprocessing
• Added two tests: One with a self-closing script tag and one with a self-closing style tag
• If a self-closing tag doesn't have any attributes it will still be ignored

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Tests

• Run the tests with npm test or yarn test)

[lukasmoellerch]

@Conduitry Thanks for pointing me in the correct direction! Is it an issue if this is only valid before preprocessing? The language server will consider this an error or is it using the preprocessor as well?

[Conduitry]

Thank you! Passing an empty string rather than undefined seems safer, so I'm going to revert your last change before merging. The documented preprocessor API currently indicates that it will pass a string as the content, and we don't want to break the existing preprocessors people have written by starting to pass undefined sometimes. Allowing preprocessors to distinguish between <script/> and <script></script> doesn't seem especially valuable anyway.

As for the language server, I'm not sure. I think that that can be configured to also use the same preprocessors as the build, but I'm not sure whether things like the TS typechecker bypass that.