Support //> options directive in compilation tests

[nicolasstucki]

This will make it simpler to copy code from issues into tests.

[som-snytt]

I gave it a half- ❤️ ed endorsement because it replaces the beloved syntax tout à fait. As a footnote, I once saw in the scala-cli docs that options is comma-separated, but I didn't verify the behavior and currently my local scala-cli doesn't start. Probably the doc is mistaken, but I haven't seen a live usage yet. Partest does something weird to remove commas:

val optionsPattern = raw"\s*//>\s*using\s+option(s)?\s+(.*)".r
              case optionsPattern(plural, rest) =>
                if (plural == null) List(rest.trim)
                else tokenize(rest).filter(_ != ",").map(_.stripSuffix(","))

grep shows one test with one-liner pragma, otherwise it's option singular in scala 2 partests.

test/files/pos/t12787.scala:// > using options -opt:inline:<sources>, -Wopt

Commas are significant in -Wconf, so quoting is required if options are comma-separated.