Dialect: add allowStarAsTypePlaceholder

[kitbellew]

This allows scala2->3 migration via kind-projector. Helps with #2807.

[tgodzik]

Is that actually supported? I am getting:

class dotty.tools.dotc.reporting.Diagnostic$Error at Section.scala:[43..44]: Not found: type *,

for:

type Combine = List[*]

[tgodzik]

Is that actually supported? I am getting:

class dotty.tools.dotc.reporting.Diagnostic$Error at Section.scala:[43..44]: Not found: type *,

for:

type Combine = List[*]

Ach, that needs an additional flag -Ykind-projector

[tgodzik]

This shouldn't be true by default, since it needs the additional flag. Do we actually need to change it in the parser? * will be treated as identifier, so everything should parse ok.

[kitbellew]

so if we return the Ident tree here, do we really need the "allow plus minus underscore Ident" flag which is really about the same but with an underscore instead? or if we need the flag, should we return the anonymous param tree then, not Ident?

[tgodzik]

I guess ident would only help with single * but not really with - and + before it. 🤔

[tgodzik]

Actually, we might need this by default anyway, since it might be problematic to add it manually for people.