Reduce the dispatching overhead in the hottest loop when parsing log-lines.

- Using a JSMap we can avoid internalizing strings
- Preprocess the dispatch table and only have varArgs or functions as
  parsers
- string[] seems to be slightly faster than string.charAt()

Bug: v8:10644
Change-Id: I03b13bdeecda1ad037191ff74e05142ceeb6533c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3571890Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79816}