feat(ui): allow conceal to be defined in decorations

Unlike syntax conceal, change highlight of concealed char

Can be used in tree-sitter using "conceal" metadata.

src/nvim/api/extmark.c

@@ -467,6 +467,11 @@ Array nvim_buf_get_extmarks(Buffer buffer, Integer ns_id, Object start, Object e
 ///                   as the mark and 'cursorline' is enabled.
 ///                   Note: ranges are unsupported and decorations are only
 ///                   applied to start_row
+///               - conceal: string which should be either empty or a single
+///                   character. Enable concealing similar to |:syn-conceal|.
+///                   When a character is supplied it is used as |:syn-cchar|.
+///                   "hl_group" is used as highlight for the cchar if provided,
+///                   otherwise it defaults to |hl-Conceal|.
 ///
 /// @param[out]  err   Error details, if any
 /// @return Id of the created/updated extmark
@@ -563,6 +568,17 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
     }
   }
 
+  if (opts->conceal.type == kObjectTypeString) {
+    String c = opts->conceal.data.string;
+    decor.conceal = true;
+    if (c.size) {
+      decor.conceal_char = utf_ptr2char((const char_u *)c.data);
+    }
+  } else if (HAS_KEY(opts->conceal)) {
+    api_set_error(err, kErrorTypeValidation, "conceal is not a String");
+    goto error;
+  }
+
   if (opts->virt_text.type == kObjectTypeArray) {
     decor.virt_text = parse_virt_text(opts->virt_text.data.array, err,
                                       &decor.virt_text_width);

src/nvim/api/keysets.lua

@@ -27,6 +27,7 @@ return {
     "number_hl_group";
     "line_hl_group";
     "cursorline_hl_group";
+    "conceal";
   };
   keymap = {
     "noremap";