Directly memcpy char* literals if they fit in the current pending
part. This avoids incremental checks for the current part size.

This will improve JSON.stringify for objects with lots of
true, false, null values by roughly 10%;

Drive-by-fix:
- Improve JSON.stringify for empty [] and {}
- Add IncrementalStringBuilder::NoExtend DECHECKs

Bug: v8:12195
Change-Id: I81ebc9e088cf983adbcfb2d768137e4a3cef9a7a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3260524Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77817}