Track depth of typed holes in lexer

[LTeuse]

In Lexer, check if we are inside a hole before parsing }> to fix issue #1301.

Resolves #1301

[jiribenes]

Thanks!

Please add a few tests into LexerTests:

effekt/effekt/jvm/src/test/scala/effekt/LexerTests.scala

Line 232 in 8acd841

test("interpolated hole string") {

(like the one from the issue)

I'm also very interested in nested holes like

<{ 
  val x = 42 + <" here's a string ">
  println(x)
  <>
}>

(and similar)