rustdoc search: allow queries to end in an empty path segment

fixes https://github.com/rust-lang/rust/issues/129707

this can be used to show all items in a module,
or all associated items for a type.
currently sufferes slightly due to case insensitivity,
so `Option::` will also show items in the `option::` module.

it disables the checking of the last path element,
otherwise only items with short names will be shown

tests/rustdoc-js-std/parser-errors.js

@@ -143,14 +143,6 @@ const PARSED = [
         returned: [],
         error: "Unexpected `:: ::`",
     },
-    {
-        query: "a::b::",
-        elems: [],
-        foundElems: 0,
-        userQuery: "a::b::",
-        returned: [],
-        error: "Paths cannot end with `::`",
-    },
     {
         query: ":a",
         elems: [],

tests/rustdoc-js-std/path-end-empty.js

@@ -0,0 +1,6 @@
+const EXPECTED = {
+    'query': 'Option::',
+    'others': [
+        { 'path': 'std::option::Option', 'name': 'get_or_insert_default' },
+    ],
+}