definite assignment analysis for let (#21024)

* draft for let daa

* patch

* fixes bugs

* errors for global let variable reassignments

* checkpoint

* out param accepts let

* add more tests

* add documentation

* merge tests

tests/init/tlet_uninit4.nim

@@ -0,0 +1,12 @@
+discard """
+  errormsg: "type mismatch: got <string>"
+"""
+
+{.experimental: "strictDefs".}
+
+proc foo(x: var string) =
+  echo x
+
+proc bar() =
+  let x: string
+  foo(x)