fix(type): preserve escaped backslash in regex string literals (#1634)

Passes `Backslash` as `escapeEscape` when scanning a regex literal, so `\\` stays a literal backslash instead of collapsing into the following character. Previously `"/^\\\\$/"` parsed to `^\$`, silently matching a literal `$`. Completes the convention arkregex already uses in `charset.ts`. Thanks to @spokodev.