underscores for routine parameters (#21192)

* underscores for routine parameters

fixes #13443, fixes #13804, refs #21121

* add changelog + more tests

* support generics and ensure inferred lambdas work

changelogs/changelog_2_0_0.md

@@ -135,6 +135,21 @@
 
 - The experimental strictFuncs feature now disallows a store to the heap via a `ref` or `ptr` indirection.
 
+- Underscores (`_`) as routine parameters are now ignored and cannot be used in the routine body.
+  The following code now does not compile:
+
+  ```nim
+  proc foo(_: int): int = _ + 1
+  echo foo(1)
+  ```
+
+  Instead, the following code now compiles:
+
+  ```nim
+  proc foo(_, _: int): int = 123
+  echo foo(1, 2)
+  ```
+
 - - Added the `--legacy:verboseTypeMismatch` switch to get legacy type mismatch error messages.
 
 ## Standard library additions and changes