Allow let to not have value when using importc (#14258)

* Allow let to not have value when using importc

This allows a let statement with the `{.importc.}` pragma to not be
initialised with a value. This allows us to declare C constants as Nim
lets without putting the value in the Nim code (which can lead to
errors, and requires us to go looking for the value). Fixes #14253

* Proper fix and documentation + changelog entry

* Improve testcase with one from timotheecour

* Add test to verify it working with macros

tests/let/timportc.nim

@@ -0,0 +1,24 @@
+discard """
+targets: "c cpp js"
+"""
+
+when defined(c) or defined(cpp):
+  {.emit:"""
+  const int TEST1 = 123;
+  #define TEST2 321
+  """.}
+
+when defined(js):
+  {.emit:"""
+  const TEST1 = 123;
+  const TEST2 = 321; // JS doesn't have macros, so we just duplicate
+  """.}
+
+let
+  TEST0 = 1
+  TEST1 {.importc, nodecl.}: cint
+  TEST2 {.importc, nodecl.}: cint
+
+doAssert TEST0 == 1
+doAssert TEST1 == 123
+doAssert TEST2 == 321

tests/let/timportc2.nim

@@ -0,0 +1,8 @@
+discard """
+  errormsg: "'let' symbol requires an initialization"
+  line: "7"
+"""
+
+# Test that this still works when not annotated with importc
+let test: cint
+echo test