Fix #9839: Drop old extension method syntax

[odersky]

No description provided.

[odersky]

I need help in changing the syntax in CB. There's simply too much of it and I am running out of time trying to fix it all. I did scalatest, utest, and xml-interpolator. I pushed a partial fix for shapeless (branch is fix-ext). @anatoliykmetyuk Can you do the others starting with the shapeless fixes?

[anatoliykmetyuk]

It seems this change messes up the parser a bit. Consider the following code:

sealed def y: Int = 1

It will report the following error:

-- Error: /Users/kmetiuk/Projects/scala3/playground/stuff/Test.scala:2:0 -------
2 |
  |^
  | values cannot be sealed

The expectation would be to report an error at line 1 and feature the code snippet in the error message. Because of this bug, I had to disable the validate-parsing neg test just to make it pass FTTB: d2da88b#diff-9a627bf280d609d3b6369aca717d6d20

[odersky]

There was indeed some breakage in Parser for modifier positions. I just fixed that one, and skipped your commits that disable tests in the new PR.