chore(parser): return statements unsupported

[joss-aztec]

Related issue(s)

Resolves #1190

Description

Summary of changes

Adds a parsing rule for return statements, such that users are shown a more informative error when they attempt to use it.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

Additional context

[kevaundray]

related to #1214

[jfecher]

validate also maps the input, so we can remove the map to return Statement::Error directly in validate:

Suggested change

	.map(|_| Statement::Error)
	.validate(|expr, span, emit| {
	emit(ParserError::with_reason(ParserErrorReason::EarlyReturnUnsupported, span));
	expr
	})
	.validate(|_, span, emit| {
	emit(ParserError::with_reason(ParserErrorReason::EarlyReturnUnsupported, span));
	Statement::Error
	})

[jfecher]

Closing in favor of #1214