feat(luahl): add priority mechanism

Base priority is 0x1000, in order to stay kinda backward compatible.
Also set tree-sitter default highlight to 100 (middle-ish value)

src/nvim/decoration.h

@@ -15,11 +15,15 @@ typedef struct {
 typedef kvec_t(VirtTextChunk) VirtText;
 #define VIRTTEXT_EMPTY ((VirtText)KV_INITIAL_VALUE)
 
+typedef uint16_t DecorPriority;
+#define DECOR_PRIORITY_BASE 0x1000
+
 struct Decoration
 {
   int hl_id;  // highlight group
   VirtText virt_text;
   // TODO(bfredl): style, signs, etc
+  DecorPriority priority;
   bool shared;  // shared decoration, don't free
 };
 
@@ -29,6 +33,7 @@ typedef struct {
   int end_row;
   int end_col;
   int attr_id;
+  DecorPriority priority;
   VirtText *virt_text;
   bool virt_text_owned;
 } HlRange;