feat(decorations): support signs

Add the following options to extmarks:
  - sign_text
  - sign_hl_group
  - number_hl_group
  - line_hl_group
  - cursorline_hl_group

Note: ranges are unsupported and decorations are only applied to
start_row

src/nvim/api/private/helpers.c

@@ -1643,3 +1643,39 @@ sctx_T api_set_sctx(uint64_t channel_id)
   }
   return old_current_sctx;
 }
+
+// adapted from sign.c:sign_define_init_text.
+// TODO(lewis6991): Consider merging
+int init_sign_text(char_u **sign_text, char_u *text)
+{
+  char_u *s;
+
+  char_u *endp = text + (int)STRLEN(text);
+
+  // Count cells and check for non-printable chars
+  int cells = 0;
+  for (s = text; s < endp; s += utfc_ptr2len(s)) {
+    if (!vim_isprintc(utf_ptr2char(s))) {
+      break;
+    }
+    cells += utf_ptr2cells(s);
+  }
+  // Currently must be empty, one or two display cells
+  if (s != endp || cells > 2) {
+    return FAIL;
+  }
+  if (cells < 1) {
+    return OK;
+  }
+
+  // Allocate one byte more if we need to pad up
+  // with a space.
+  size_t len = (size_t)(endp - text + ((cells == 1) ? 1 : 0));
+  *sign_text = vim_strnsave(text, len);
+
+  if (cells == 1) {
+    STRCPY(*sign_text + len - 1, " ");
+  }
+
+  return OK;
+}