Port action templates functionality to ANTLR5

[DavidGregory084]

This PR ports the action templates functionality from https://github.com/antlr/antlr4/pull/4345 to ANTLR5.

This enables users to write cross-target grammars which make use of actions, by enabling users to provide action templates as StringTemplate .stg group files on the command line.

By providing different action templates for each target language, users can provide a different implementation of the action logic for each target.

Java.stg:

normalizerImports() ::= <<
import java.text.Normalizer;
import java.text.Normalizer.Form;
>>
normalize(s) ::= <<Normalizer.normalize(<s>, Form.NFKC)>>
getText() ::= <<getText()>>
setText(s) ::= <<setText(<s>);>>

Javascript.stg:

normalizerImports() ::= ""
normalize(s) ::= <<<s>.normalize("NFKC")>>
getText() ::= "this.text"
setText(s) ::= "this.text = <s>"

The example below is the motivating example for me - I have a grammar that I'd like to use in both a JVM-based compiler and a VS Code extension:

Example.g4:

lexer grammar Example;

@lexer::header {
<normalizerImports()>
}

ID : (ID_START ID_CONTINUE* | '_' ID_CONTINUE+) { <setText(normalize(getText()))> } ;
ID_START : [\p{XID_Start}] ;
ID_CONTINUE: [\p{XID_Continue}] ;
WS : (' '|'\n') -> skip ;