Convert getLeadingTrivia to a getter

[TwitchBronBron]

Now that #1225 landed, I think we should convert .getLeadingTrivia() into a getter. This way it is easier for plugins to manipulate the AST without needing to call a function to get the results, and makes it more clear that it's an attribute of a node rather than a return value.

Tasks:

• convert all .getLeadingTrivia() methods into get leadingTrivia()
• add unit tests to prove we can manipulate the leading trivia and it transpiles properly. something like:

const file = program.setFile<BrsFile>('source/main.brs', `
    function test()
    end function
`);
file.parser.ast.walk(()=>{
    FunctionStatement: (stmt) => {
        editor.arrayPush(stmt.leadingTrivia, createCommentToken(''Hello world");
    }
});
testTranspile(.....)