Add a new Fortran grammar

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Looks like the Fortran grammar was removed a few years ago (#111) (because the repository was deleted and grammar removed) but we could use grammars from https://github.com/fortran-lang/vscode-fortran-support/tree/main/syntaxes instead?

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Trying to make a PR for this but I have a dilemma for sorts... there are two grammars for Fortran, the fixed form grammar and the free form grammar. Fixed form is for older versions of fortran, and free form for the modern versions.

This is what I have right now, with the free form grammar:

export const githubGrammarSources = {
  // ...
  ['fortran', 'https://github.com/fortran-lang/vscode-fortran-support/blob/main/syntaxes/fortran_free-form.tmLanguage.json'],
}
export const languageAliases = {
  // ...
  fortran: ['f90', 'f95', 'f03', 'f08', 'f18', 'fpp'],
}

But I'm wondering if I should (also) do something like this for the fixed form:

export const githubGrammarSources = {
  // ...
  ['fortran-fixed', 'https://github.com/fortran-lang/vscode-fortran-support/blob/main/syntaxes/fortran_fixed-form.tmLanguage.json'],
}
export const languageAliases = {
  // ...
  'fortran-fixed': ['f', 'f77', 'for'],
}

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I gave it a shot a while back and I couldn't get it to properly syntax highlight. I would appreciate if someone else could take a look!

[GitMensch]

Like with COBOL it would be good to have separate grammars. If I remember correct a grammar can include/extend another one, which then would be a reasonable thing to do here.

[antfu]

Hey, thanks for your contribution! With the recent rewrite of v1.0, we have moved the grammars out of this repository. We generally suggest you loading the grammar locally to move forward. If you believe this is the language that is widely used, you can send an issue or PR to this repo: https://github.com/shikijs/textmate-grammars-themes

Thank you!

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I've added it in shikijs/textmate-grammars-themes#18 :)