Only warn about missing features for mainstream modern browsers

[lionel-rowe]

Closes #489:

IE is officially dead, so maybe these warnings should be removed for features supported in all current browsers, such as the u flag.

Methodology: check caniuse for each feature, only including mainstream modern browsers (no IE, no Opera Mini, and ignoring the various Chinese browsers, which are all based on mainstream modern browser engines).

Sources:

• neglookbehind, poslookbehind: https://caniuse.com/js-regexp-lookbehind
• sticky: https://caniuse.com/mdn-javascript_builtins_regexp_sticky
• unicode: https://caniuse.com/mdn-javascript_builtins_regexp_unicode
• dotall: https://caniuse.com/mdn-javascript_builtins_regexp_dotall
• unicodecat, unicodescript: https://caniuse.com/mdn-javascript_builtins_regexp_property_escapes
• namedgroup: https://caniuse.com/mdn-javascript_builtins_regexp_named_capture_groups

Results: only neglookbehind and poslookbehind are missing, thanks to iOS Safari. Presumably upcoming features like RegExp v flag will also lack comprehensive support for a while after they're officially added to the spec.

[gskinner]

@lionel-rowe - thanks for this PR. Good timing too, as I'm currently planning to jump back into RegExr maintenance / updates beginning early next week, after a long hiatus. I'll take a look then, and let you know if I have any specific feedback, but at a glance this looks good.

[lionel-rowe]

@gskinner Even better timing — I just noticed https://caniuse.com/js-regexp-lookbehind has been updated, and it looks like iOS Safari 16.4 will ship with support for neglookbehind and poslookbehind! 🎉

Will leave PR as-is for now though as iOS Safari 16.4 is still in beta.