URLPattern: Emit {} when plain text follows a :foo group.

This CL fixes another problem case from:

whatwg/urlpattern#145

In this case we need to detect when fixed text following a `:foo` group
could be mistaken as party of the name.  When this happens we should
emit `{ }` braces around the group to isolate it.  For example,
`{:foo}bar` instead of `:foobar`.

Bug: 1263673
Change-Id: I9c9bfb736c6e8880bceeae9280f0fc30f794cea8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3321520
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/main@{#950813}

urlpattern/resources/urlpatterntestdata.json

@@ -2565,5 +2565,22 @@
     "expected_match": {
       "pathname": { "input": "foobar", "groups": { "foo": "foo" }}
     }
+  },
+  {
+    "pattern": [{ "pathname": "{:foo}bar" }],
+    "inputs": [{ "pathname": "foobar" }],
+    "expected_match": {
+      "pathname": { "input": "foobar", "groups": { "foo": "foo" }}
+    }
+  },
+  {
+    "pattern": [{ "pathname": ":foo\\bar" }],
+    "inputs": [{ "pathname": "foobar" }],
+    "expected_obj": {
+      "pathname": "{:foo}bar"
+    },
+    "expected_match": {
+      "pathname": { "input": "foobar", "groups": { "foo": "foo" }}
+    }
   }
 ]