compiler/suggest: add variable support to con (#12569)

This allows for the type of a variable to be retrieved.

nimsuggest/tests/tcon_variable.nim

@@ -0,0 +1,12 @@
+let foo = "string"
+var bar = "string"
+bar#[!]#.add foo
+bar.add foo#[!]#
+
+discard """
+$nimsuggest --tester $file
+>con $1
+con;;skVar;;tcon_variable.bar;;string;;$file;;2;;4;;"";;100
+>con $2
+con;;skLet;;tcon_variable.foo;;string;;$file;;1;;4;;"";;100
+"""