Scaladoc - fix uncompiling snippets, fix indentation for snippets

[rochala]

Main change of this PR is fixing existing snippets and adding new flag for macro snippets to fix it's creation.
The repair of it was due, mainly for #10896 ( this issue, may or may not be closed after this PR, as it's unresolved )

First one is sc:macrocompile which allows to parse macros as snippets. Before this changes, snippets were wrapped in traits which are not static what lead to macro malformation. This flag skips wrapping in Traits with self types in favour of single object Snippet { ... } wrapping to enable snippet validation at compile time.

https://dotty.epfl.ch/api/scala/quoted/Varargs$.html#

Before:

After ( + it now checks snippet at compiletime ):

[pikinier20]

Basically, we started with idea to add macrocompile flag but ended up with getting rid of the context-providing feature in code snippets. The feature wrapped snippets in traits with the self-type set up to the class in which the snippet is defined. This was meant to help author with writing snippet without the need to import and instantiate stuff. However, we can do it by ourselves if we need that and just hide the unnecessary boilerplate so that user sees only relevant part.

Removing this feature solved the problem with macros as well as with running snippets in API docs. It was also based on some nasty piece of code that we couldn't prove it actually works.

[pikinier20]

As it affects snippets in stdlib documentation, I'll ask @nicolasstucki if he's fine with that.

[KacperFKorban]

It seems that most of the changes are just adding a wildcard import from the current package. Can we maybe add it as default to the generated code?