This patch modifies irregexp's error handling. Instead of representing
errors as C strings, they are represented as an enumeration value
(RegExpError), and only converted to strings when throwing the error
object in regexp.cc. This makes it significantly easier to integrate
into SpiderMonkey. A few notes:

1. Depending on whether the stack overflows during parsing or
   analysis, the stack overflow message can vary ("Stack overflow" or
   "Maximum call stack size exceeded"). I kept that behaviour in this
   patch, under the assumption that stack overflow messages are
   (sadly) the sorts of things that real world code ends up depending
   on.

2. Depending on the point in code where the error was identified,
   invalid unicode escapes could be reported as "Invalid Unicode
   escape", "Invalid unicode escape", or "Invalid Unicode escape
   sequence". I fervently hope that nobody depends on the specific
   wording of a syntax error, so I standardized on the first one. (It
   was both the most common, and the most consistent with other
   "Invalid X escape" messages.)

3. In addition to changing the representation, this patch also adds an
   error_pos field to RegExpParser and RegExpCompileData, which stores
   the position at which an error occurred. This is used by
   SpiderMonkey to provide more helpful messages about where a syntax
   error occurred in large regular expressions.

4. This model is closer to V8's existing MessageTemplate
   infrastructure. I considered trying to integrate it more closely
   with MessageTemplate, but since one of our stated goals for this
   project was to make it easier to use irregexp outside of V8, I
   decided to hold off.

R=jgruber@chromium.org

Bug: v8:10303
Change-Id: I62605fd2def2fc539f38a7e0eefa04d36e14bbde
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2091863
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66784}