store full definition AST for consts, fix noRewrite (#20115)

* continue #9582 for consts, close #9331, fix #20114

also move extractPragma to ast to pave the way for things like {.strdefine: "abc".} etc

* changelog correctly

* fix jsgen

* update tgetimpl

* fix sighashes

* fix #19766, add comment about postfix

* fix noRewrite LOL

refs #16620

* fix changelog

* fix destructors

tests/macros/t19766_20114.nim

@@ -0,0 +1,16 @@
+discard """
+  action: compile
+  nimout: '''
+const
+  foo {.strdefine.} = "abc"
+let hey {.tddd.} = 5
+'''
+"""
+
+import macros
+
+template tddd {.pragma.}
+
+expandMacros:
+  const foo {.strdefine.} = "abc"
+  let hey {.tddd.} = 5

tests/macros/tgetimpl.nim

@@ -1,5 +1,5 @@
 discard """
-  nimout: '''"muhaha"
+  nimout: '''foo = "muhaha"
 proc poo(x, y: int) =
   let y = x
   echo ["poo"]'''