Consider searching with JS regexes in unicode mode

[roblourens]

JS regexes now support a u flag which changes their semantics regarding unicode characters in some ways and enables some new features.

Examples:

/𝌆{2}/u.test('𝌆𝌆') => true
/𝌆{2}/.test('𝌆𝌆') => false

/\p{Sm}/u.test('+') => true
/\p{Sm}/.test('+') => false

Example in vscode:

• Have 𝌆𝌆 in editor
• Search 𝌆{2}
• No match
• Searching 𝌆𝌆 does match

Not sure how many users are affected by this but it's worth investigating.

Also, this support seems half-baked in Node 8 but works more in Node 10, so maybe we wait for an electron update.

More info: https://mathiasbynens.be/notes/es6-unicode-regex

[roblourens]

@rebornix now that we are using Node 10, I suggest we do this for the editor. Any reason not to?

[rebornix]

@roblourens I'm not aware of any reason why we don't want to do that. If I understand correctly, we only need to add u flag to the regex search, right?

[roblourens]

Yes. I suggest we move it to August to give plenty of time. This goes with #62416 too.

[roblourens]

Broke some tests, couldn't get them fixed in time, pushing to september.