return statement parsing needs to be more restrictive

[nofun97]

Purpose

To get a better resulting protobuf structure from return statements.

Problem

Currently, the grammar for return statement is

ret_stmt        : RETURN TEXT;

which basically just parses anything next to the token return. The resulting json of the return statement is

{
       "ret": {
        "payload": "Anything next to the word return"
       }
}

This makes it difficult to extract any information from the return statement. This also removes the ability to add modifiers which is allowed in the sysl grammar.

statements: ( ...
                | ret_stmt
                ...
            )
            attribs_or_modifiers?
            ;

Suggested approaches

Allow a more restrictive grammar to return statements. Something like the following:

status_code: [2-5][0-9][0-9];
ret_stmt: RETURN (OK | ERROR | status_code) LESS_COLON TEXT;

The token TEXT above needs to capture everything and stops until a square bracket to make way for attribs_or_modifiers rule.

The structure for return statement also needs to be improved.

type Return struct {
	state         protoimpl.MessageState
	sizeCache     protoimpl.SizeCache
	unknownFields protoimpl.UnknownFields

	Payload string `protobuf:"bytes,1,opt,name=payload,proto3" json:"payload,omitempty"`
}

it needs to capture the type of return statement (ok, error, and status_code) and the payload (what is being returned).

[orlade-anz]

Not P0 since we can work around this in post-processing for the time being.