Replace ANTLR with new parser

[bioball]

The current parser is quite slow. On my machine (macOS on M1 Max), it takes almost half a second to parse pkl:base in Java.

In some large projects, about 40% of the wall clock time is taken by ANTLR parsing.

It's time to replace it with a hand-rolled parser.

[sgammon]

This will be big for a few other reasons too:

Sometimes ANTLR is shaded with Truffle builds. The version of ANTLR they provide conflicts with the fork Pkl prefers; in particular, the fork uses ANTLR's gencode v3, and GVM ships a version using v4. In particular, Sulong (GraalVM's LLVM interpreter) depends on the shaded copy.

Additionally, this is one non-modular dependency eliminated on the way toward true JPMS support, which would enable shipping a Pkl jmod and usage in jpackage and jlink targets

[bioball]

ANTLR also generates grammars that can only be used with that version of ANTLR (unless this has changed recently).

If the runtime library differs from the version used to generate your grammar, you have a conflict. That's partially why ANTLR gets shaded so often.

[driverkt]

Just an observer, but would it be worth considering javacc? There isn't a list of requirements here, per se.

Update: Looks like work has already begun on a custom parser. You may disregard this suggestion.