Intent to ship: RegExp escaping proposal

[Daniel Minor]

As of Firefox 134, I intend to turn the RegExp escaping proposal implementation on by default on all platforms. It has been developed behind the `javascript.options.experimental.regexp_escape` preference. The proposal is currently at Stage 3 of the TC39 process, Chromium has patches under development at: https://chromium-review.googlesource.com/c/v8/v8/+/5768527, Safari shipped their implementation in 18.2: https://developer.apple.com/documentation/safari-release-notes/safari-18_2-release-notes#JavaScript.

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1918235 is the bug to enable.

Standard: https://github.com/tc39/proposal-regex-escaping

Daniel