Allow nesting of brace-like delimiters in regex

Allow nesting of brace-like delimiters in regex

Summary:
Allow nested brace-like delimiters in regular expressions.
Check balanced nested delimiters only for delimiter type on outside.
Done in 2 steps:

1 ) Find ending delimiter by searching from end of regex
  - if cannot find ending, report Missing delimiter

2 ) Take inner string (w/o outer delimiters) and use a stack to push and pop delimiters in linear scan across regex. Report missing delimiter if cannot find corresponding delimiter.

  - skip escaped delimiters
  - re"[[]]" still reports an error: ```Bad regex pattern; missing terminating ] for character class [2]. (Typing[4275])``` However, it passes through check of balanced brace delimiters, and the error is thrown farther along in the regex check. New error appears to be legitimate.

Reviewed By: kmeht

Differential Revision: D16182873

fbshipit-source-id: 012b677df29d415dfd7bc57e3f643d4eac86edad