Improve parsing when substitution is in play

[meck-gd]

This MR:

• Improves parsing when characters like " or ^ come in through a variable substitution.
• Prevents replacing commas and semicolons when a variable is referenced. This caused problems with a script that stored a PS script chunked into hundreds of variables and then concatenated all of them.

The comma/semicolon thing is a bit fishy in general - normalizing something like echo a,b currently also incorrectly yields echo a b, but I don't have a good solution because in some cases it is wanted (test_comma_semi_colon).