Add DSP.

[modulovalue]

Fixes #54540.

Other changes:

• There's a new FAQ section that describes how to generate parsers.
• I've dedented the custom code because ANTLR adds its own indentation to generated parsers.

Also, just so that we don't forget this, it would be great to maintain the git history of Dart.g, i.e., if Dart.g is removed, it might be better to replace it with the contents of Dart.g4 in this PR instead of just removing Dart.g and keeping this Dart.g4.

[copybara-service]

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[devoncarew]

@eernstg - it looks like you've started the review in https://dart-review.googlesource.com/c/sdk/+/349408. If it looks good, I believe you'll need a 2nd reviewer to lgtm it.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/349408 has been updated with the latest commits from this pull request.

[eernstg]

Hi @devoncarew, thanks for taking a look at this! I actually landed the CL without problems. So now we just need a newer ANTLR on all systems. ;-)