Generated accessors for optional items should allow undefined

[sharwell]

As described in #261, the generated accessor AContext.b() has the wrong return type for the following grammar:

grammar Test;

a : b? c d* d+ ;
b : ID;
c : ID;
d : ID;
e : ID;

Expected signatures:

export class AContext extends ParserRuleContext {
  b(): BContext | undefined;
  c(): CContext;
  d(): DContext[];
  d(i: number): DContext;
  e(): EContext[];
  e(i: number): EContext;
  // ...
}

Actual signatures:

export class AContext extends ParserRuleContext {
  b(): BContext;
  c(): CContext;
  d(): DContext[];
  d(i: number): DContext;
  e(): EContext[];
  e(i: number): EContext;
  // ...
}

Difference we want to see (actual → expected)

 export class AContext extends ParserRuleContext {
-  b(): BContext;
+  b(): BContext | undefined;
   c(): CContext;
   d(): DContext[];
   d(i: number): DContext;
   e(): EContext[];
   e(i: number): EContext;
   // ...
 }

/cc @mike-lischke

[sharwell]

📝 Currently blocked on tunnelvisionlabs/antlr4#15.