Add clampToContainer property for containing-block clamping

Expose a reflected clampToContainer boolean that, when set, makes fit()
clamp the popup against its nearest containing-block rect instead of the
viewport. Required for popups under an ancestor with transform/filter/
contain:layout|paint, where position: fixed resolves against that ancestor.

Bumps @tp/helpers dependency to ^2.13.0 for the matching _posFixed support.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@tp/tp-popup",
-  "version": "1.3.0",
+  "version": "1.4.0",
   "description": "",
   "main": "tp-popup.js",
   "scripts": {
@@ -13,7 +13,7 @@
   "author": "trading_peter",
   "license": "Apache-2.0",
   "dependencies": {
-    "@tp/helpers": "^2.0.0",
+    "@tp/helpers": "^2.13.0",
     "@tp/tp-icon": "^1.0.1",
     "lit": "^3.0.0"
   }