Endless methods

[aibaars]

This pull request adds "endless" method definitions. Ruby 3 allows for method definitions like

def square(x) = x * x

See also https://bugs.ruby-lang.org/issues/16746

Checklist:

• All tests pass in CI.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

The STATE_COUNT is increased, hopefully not excessively:
before:

src/parser.c:#define STATE_COUNT 3312
src/parser.c:#define LARGE_STATE_COUNT 1237

after:

src/parser.c:#define STATE_COUNT 3856
src/parser.c:#define LARGE_STATE_COUNT 1453
```

[maxbrunsfeld]

Looks great