custom integer literals (#17489)

* user defined integer literals; refs #17020
* updated renderer.nim
* use mlexerutils helper
* imported all test cases from https://github.com/nim-lang/Nim/pull/17020
* final grammar updated

tests/lexer/mlexerutils.nim

@@ -0,0 +1,9 @@
+import macros
+
+macro lispReprStr*(a: untyped): untyped = newLit(a.lispRepr)
+
+macro assertAST*(expected: string, struct: untyped): untyped =
+  var ast = newLit(struct.treeRepr)
+  result = quote do:
+    if `ast` != `expected`:
+      doAssert false, "\nGot:\n" & `ast`.indent(2) & "\nExpected:\n" & `expected`.indent(2)