fix bugs with dot & call operators [backport] (#20931)

* better error messages for dot operators [backport]

fixes #13063

* also fixes #7777

* fix #6981 and #9831 too

* fix

* minor improvement

* sus test fixes

* make test multiplatform lol

* fix nimsuggest test, extra improvements

tests/specialops/tnewseq.nim

@@ -0,0 +1,22 @@
+# issue #6981
+
+import std/assertions
+
+{.experimental: "callOperator".}
+
+block: # issue #6981
+  proc `()`(a:string, b:string):string = a & b
+
+  var s = newSeq[int](3)
+
+  doAssert s == @[0, 0, 0]
+
+block: # generalized example from #6981
+  proc mewSeq[T](a: int)=discard
+  proc mewSeq[T]()= discard
+  mewSeq[int]()
+
+block: # issue #9831
+  type Foo = object
+  proc `()`(foo: Foo) = discard
+  let x = newSeq[int]()