add rule context object factory?

``` java
public static class LExprBaseContextFactory {
public SContext createS(ParserRuleContext parent, int state) {
return new SContext(parent, state);
}
public EContext createE(ParserRuleContext parent, int state, int _p) {
return new EContext(parent, state, _p);
}
public MultContext createMult(EContext parent) { return new MultContext(parent); }
public AddContext createAdd(EContext parent) { return new AddContext(parent); }
public IntContext createInt(EContext parent) { return new IntContext(parent); }
}
```

Start by investigating how generated parser rule context classes are currently constructed, using the LExprBaseContextFactory example in the issue as the starting point. The issue names no files, tests, entry points, or acceptance criteria, so the intended factory design and definition of done require clarification before implementation.

Written by the indexing model from the issue text.