string interpolation identifier character blacklist

[StefanKarpinski]

From #25178:

The minimal change for 1.0 is to implement an identifier blacklist and only allow bare name interpolation when the identifier is not followed by something not on the blacklist (i.e. followed by either a blacklisted character or the end of the string). I kind of like the idea of the $ prefix for $-interpolating strings, but I'm not that against just continuing with $ unprefixed.

@stevengj suggested as an initial blacklist:

Operator characters, #, whitespace, and most punctuation

This should be done for 1.0 but does not need to be done right away since this would affect very little code in the wild.