Fix incompatibilities with Golang parser generation

[mdehoog]

Generating a Golang antlr4 parser from the grammar is currently broken due to some idiosyncratic language incompatibilities:

• boolean -> bool is not supported (upstreaming a fix here: github.com/Add support to Golang codegen for booleans antlr/antlr4#4231)
• Golang generation generates both a NewExpressionContext struct and function in the same package, causing a redeclaration error
• start / end indexing conflicts with Antlr4 Golang runtime GetStart and GetEnd methods

This PR fixes the second two by:

• Renaming NewExpression to NewExpr which changes the NewExpressionContext type to NewExprContext
• Renaming start and end to startIndex and endIndex

[github-actions]

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[ekpyron]

I don't see any harm in these changes, thanks!