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.nim

@@ -0,0 +1,34 @@
+{.experimental: "strictDefs".}
+
+proc bar(x: out string) =
+  x = "abc"
+
+proc foo() =
+  block:
+    let x: string
+    if true:
+      x = "abc"
+    else:
+      x = "def"
+    doAssert x == "abc"
+  block:
+    let y: string
+    bar(y)
+    doAssert y == "abc"
+  block:
+    let x: string
+    if true:
+      x = "abc"
+      discard "abc"
+    else:
+      x = "def"
+      discard "def"
+    doAssert x == "abc"
+  block: #
+    let x: int
+  block: #
+    let x: float
+    x = 1.234
+    doAssert x == 1.234
+static: foo()
+foo()