Fix edge case issues with replacement text syntax

[slevithan]

In XRegExp replacement text syntax, all of the following should throw errors but currently don't:

• Unescaped $ at the end of the string (no following character).
• $ (dollar space).
• Named backferences with leading or trailing spaces, e.g. $< name> or ${name }.
• Numbered backferences using brace syntax with leading or trailing spaces, e.g. $< 1> or ${1 }.