Make "unreachable statement" error a warning

[Yardanico]

Currently code like

proc a() = 
  quit(0)
  echo "hi"

a()

would not compile with the error unreachable statement after 'return' statement or '{.noReturn.}' proc. I propose to make it a warning instead, which will still be quite visible to the developer but it'll allow for faster prototyping in some cases.

Please comment, I might not know some cases where this can lead to new bugs or unintended behaviour.

Also see nim-lang/Nim#14190 because it's a bit similar in a sense that this PR made it possible to have unreachable else branch in a case statement.

[timotheecour]

'If true: quit' is good enough

[Araq]

Yeah, however, if true is an idiom that has to be learned. I'm torn.

[simonkrauter]

+1
This would make it easier and more beginner friendly, for example when inserting an early return statement for debugging purpose.

[juancarlospaco]

when off: ?.

[Clyybber]

This:

proc a() = 
  quit(0)
  block: echo "hi"

a()

currently compiles without error, which is a bit illogical, but if we were to fix it so that it errors it would be a breaking change.
Making it a warning and fixing it would be a solution without a breaking change (I did that in nim-lang/Nim#14816) :D