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_uninit3.nim

@@ -0,0 +1,24 @@
+discard """
+  cmd: "nim check $file"
+  action: "reject"
+  nimout: '''
+tlet_uninit3.nim(13, 5) Error: 'let' symbol requires an initialization
+tlet_uninit3.nim(19, 5) Error: 'x' cannot be assigned to
+tlet_uninit3.nim(23, 11) Error: 'let' symbol requires an initialization
+'''
+"""
+
+{.experimental: "strictDefs".}
+
+let global {.used.}: int
+
+proc foo() =
+  block:
+    let x: int
+    x = 13
+    x = 14
+
+  block:
+    let x: int
+    doAssert x == 0
+foo()