change \uXXXX \UXXXXXX string literal escape syntax to \u{}

[andrewrk]

Extracted from #2097 (comment)

This matches (thanks @daurnimator for doing the research):

• javascript (since ES6)
• lua (since 5.3)
• swift (who swapped from our current syntax!)
• rust

[binary132]

Why are braces necessary for Unicode but not for hexadecimals? This feels inconsistent.

[Hejsil]

@binary132 Probably because \u can be variable length, so we need a way to specify where the end of the escape sequence is. Is \u000000 \u{0000}00 or \u{000000}?

[andrewrk]

For this particular syntax, cross language consistency is more valuable than self consistency.

Hexadecimal escapes are always 2 digits. Unicode escapes used to always be 4 digits. But then 6 became possible with changes to Unicode. So here we are.