Simplify parser

Simplify parser

The parser distinguishes between var_decl, top_var_decl, and top_var_decl_no_assign even though the AST does not. There's no logical need for the grammar to have three separate rules; semantic check could handle the potential syntax errors.
For instance, Stan does not allow tilde-statements outside of the model block but there's no special statement_no_tilde rule in the grammar for other blocks. So this fails to parse

parameters {
  real x;
  x ~ std_normal(); // only decls allowed
}

but this parses successfully and fails in semantic check

parameters {
  real x;
}
generated quantities {
  x ~ std_normal(); // any statement parses
}

Parser errors are defined in parser.messages. That file is not easy to work with. Currently it is incomplete because I didn't know I was supposed add some error states there in PR #573
The parser error messages have no context other than the parser state that failed. We decided to fix #469 by changing the error message. The message relevant is here:
https://github.com/stan-dev/stanc3/blob/458a7933728debdb8b3ce14e5af6960faef95513/src/frontend/parser.messages#L1392-L1402
The parser state 661 corresponds to any unexpected symbol in data or parameters block. As far as I know the only way to give a specific error for a semicolon is by creating a new parser state with the UNREACHABLE hack. On the other hand, if this were handled in Semantic_check the change would be trivial.

So I'm proposing changing the parser to accept any program that AST can represent and moving all other checks to Semantic_check.

@rybern what do you think?

Start by comparing the grammar rules for var_decl, top_var_decl, and top_var_decl_no_assign with parser.messages and the Semantic_check entry points. Review the examples in this issue and the linked discussions for #469, #573, and #581; done means AST-representable programs parse while remaining restrictions are reported by semantic checking, with relevant parser messages updated.