enh(parser) Compiler extensions (for grammars + plugins)

[joshgoebel]

Closes #2794. Works towards #2520. Closes #2833.

Changes

Implements #2833 with the key compilerExtensions on a grammar which is an
array of extensions that takes the mode (and it's parent) as input.

Inside the compiler (for all grammars):

• More modular pipeline that users the new extensions internally also
• Adds support for illegal being an array of illegal expressions seamlessly compiled into a single regex

R grammar:

• Testing out the beforeMatch (alternative to look-behind) as a candidate for future inclusion in the compiler.

It also adds two plugin hooks that take the same input:

• before:compileEarly
• before:compileLate

The plugin hooks were removed for now.

Overall this allows grammars (and plugins) to add syntactic sugar to their own definitions.

Checklist

• Added markup tests, or they don't apply here
• Added tests to guarantee plugin extensions are called properly by compiler
• Updated the changelog at CHANGES.md
• Added myself to AUTHORS.txt, under Contributors
• Document the new plugin options

[joshgoebel]

Ping.

[allejo]

This looks good to me. Some comments have become dated though and should be updated for accuracy

[egor-rogov]

It's really cool. The feature can be easily abused, but it's a powerful tool.
I mentioned some points regarding the documentation.

[egor-rogov]

There are actially before:compileEarly and before:compileLate, right?

[joshgoebel]

I need to rework all the docs (which is why this isn't merged). No one has asked for this as a plugin so I'm probably going to rip that out completely (plus I'm not sure a world where to install grammar xyz you also need plugin 123 and 234 is a great place to end up, so we can push off that decision into the future)... so the new capability is just that languages can define their own sugar (and of course the more modular compiler setup)... and then inside the library we can "test" new extensions in a single language first... and if that goes well they can be promoted to the compiler.

So these docs are probably moving into compilerExtensions inside the mode docs.

[joshgoebel]

Moved docs into the mode reference.

[egor-rogov]

I really like it.
The feature is great, the added docs are clear.