Improve support for Unicode supplementary characters in identifiers and string interpolation

[som-snytt]

Precedence uses codepoint when probing lead char.

Scanner accepts supplementary chars in more places,
such as op_Supple, 'Supple'.

Fixes scala/bug#12482
Fixes scala/bug#12484
Fixes scala/bug#12485
Fixes scala/bug#12486

[som-snytt]

While looking at this, I had to relearn scala/vscode-scala-syntax#222 (comment)

in fact the last thing in my repl history is:

scala> implicit class P(sc: StringContext) { def x_*(parts: Any*) = "ha" }
class P

scala> x_*"no"
val res0: String = ha

Conversely, the obsolete test code was going to test s"$x_*" which is not allowed.

[martijnhoekstra]

I promise I'll get around to this this week, sorry for taking so long.

[SethTisue]

@martijnhoekstra still interested in pursuing this?

[som-snytt]

They have really long vacations over there. In America, they talk about the Halloween to New Year's period, but there it's All Hallow's to Summer Solstice.

[SethTisue]

@unkarjedy you opened all four of the original tickets; would you be interested in reviewing this?

[som-snytt]

Also thanks @martijnhoekstra for previous help.

As the author, I am off the hook for review, which would require me to understand the delta. I dread the forward port, if necessary.

[som-snytt]

I was about to say thanks @martijnhoekstra but I see I just said that. Now I have to review how this PR compares with what is on dotty.

[lrytz]

LGTM!

[SethTisue]

@som-snytt shall we go ahead and hit "merge"? can't quite tell if you're done.

[som-snytt]

I'm not sure if @lrytz is done. Edit: I'm not sure if lrytz will say I'm not done.

[lrytz]

Happy to have it merged, my comment was just a nudge to see if you have a better idea.

[som-snytt]

Not only am I out of ideas, I have no memory of what this code does. If I have an idea when forward porting to Scala 3, I'll be sure to backport it.