Add body field for end-less methods

[aibaars]

224 added a body field for method bodies. However, we forgot about end-less methods., such as:

def foo = bar
def Object.foo = bar

As a result their bodies are still represented as unnamed children.

@npezza93 @maxbrunsfeld

Checklist:

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