Adds nvim_get_hl_by_name/by_id

...in order to retrieve highlights.

Added test/functional/api/highlight_spec.lua
HL_NORMAL is not really a good name, since it's more like an empty attribute than the normal's one.
If one pays attention, syn_cterm_attr2entry is never called with attr=0 because it's always special cased before.
I suggest in subsequent PRs we remove the ATTR_OFF and just insert an EMPTY ATTR/RESET_ATTR/UNINITIALIZED for id 0.

src/nvim/ui.h

@@ -21,6 +21,9 @@ typedef struct {
   int foreground, background, special;
 } HlAttrs;
 
+#define HLATTRS_INIT \
+  ((HlAttrs){ false, false, false, false, false, -1, -1, -1 })
+
 typedef struct ui_t UI;
 
 struct ui_t {