implements https://github.com/nim-lang/RFCs/issues/258 (#15503)

* implements https://github.com/nim-lang/RFCs/issues/258

* don't be too strict with custom pragma blocks

* cast pragmas: documentation

* added most missing inference query procs to effecttraits.nim

tests/effects/tcast_as_pragma.nim

@@ -0,0 +1,18 @@
+discard """
+  cmd: "nim c $file"
+  action: "compile"
+"""
+
+proc taggy() {.tags: RootEffect.} = discard
+
+proc m {.raises: [], tags: [].} =
+  {.cast(noSideEffect).}:
+    echo "hi"
+
+  {.cast(raises: []).}:
+    raise newException(ValueError, "bah")
+
+  {.cast(tags: []).}:
+    taggy()
+
+m()