[BUGFIX] Solve two bugs with string interpolation

[NotHyper-474]

Overview

• Quote escapes now works properly inside ${...} expressions.
  • The inner expression will now also be isolated (as if enclosed in ()) only if necessary.
• Identifiers in expressions like $a$b now get correctly interpreted as strings if those happen to be numbers.

Updated Assert Script

import funkin.modding.module.Module;
import funkin.modding.module.ModuleHandler;

class StrInterpAssert extends Module
{
  override function onCreate(_)
  {
    var x = 10;

    function test(s:String)
    {
      return s;
    }

    var ease:String = 'cubic';
    var easeDir:String = 'Out';

    var a = 1;
    var b = 2;

    assertStr("$a$b", '$a$b', "12");
    assertStr("${'$a$b'}", '${'$a$b'}', "12");
    assertStr("$ease$easeDir", '$ease$easeDir', "cubicOut");
    assertStr("$ease${easeDir}", '$ease${easeDir}', "cubicOut");
    assertStr("${ease}$easeDir", '${ease}$easeDir', "cubicOut");
    assertStr("${'{'}", '${'{'}', "{");
    assertStr("${'}'}", '${'}'}', "}");
    assertStr("${'{}'}", '${'{}'}', "{}");
    assertStr("${\"{\"}", '${"{"}', "{");
    assertStr("${test('string')}", '${test('string')}', "string");
    assertStr("${test('string ${0}')}", '${test('string ${0}')}', "string 0");
    assertStr("$x and ${test('$x') + '10'}", '$x and ${test('$x') + ' 10'}', "10 and 10 10");
    assertStr("$$", '$$', "$");
    assertStr("$$$x", '$$$x', "$10");
    assertStr("$$$x ", '$$$x ', "$10 ");
    assertStr("$-=", '$-=', "$-=");
    assertStr("$x ", '$x ', "10 ");
    assertStr("$x", '$x', "10");
    assertStr("$", '$', "$");
    assertStr("${'Escaped 'single quoted' string.'}", '${'Escaped \'single quoted\' string.'}', "Escaped 'single quoted' string.");
    assertStr("$ x", '$ x', "$ x");
    assertStr("$x + 3 is ${x + 3}", '$x + 3 is ${x + 3}', "10 + 3 is 13");
  }

  function assertStr(literal:String, str:String, ass:String)
  {
    if (literal?.length == 0) throw 'Forgot "literal"!';
    if (str?.length == 0) throw 'Forgot "str"!';
    if (ass?.length == 0) throw 'Forgot "ass"!';
    trace('ASSERT: ("' + literal + '" -> "' + ass + '"), RESULT: "' + str + ((str == ass) ? '" (SUCCESS)' : '" (FAIL)'));
  }
}

The strings in the script look like this after parsed (some of these traces come from nested strings):