Regex: share the PCRE2 wrapper beyond adblock

Compiled patterns are useful anywhere someone else writes the pattern:
the adblock lists today, agent tool arguments next. The wrapper moves
out of the adblock directory and gains an options struct (case, UTF-8
subjects) and a compile diagnostic the caller can log or show. The App
owns the one context every consumer compiles through, the blocker
included.

src/network/HttpClient.zig

@@ -4427,7 +4427,7 @@ test "HttpClient: adblock verdicts apply per request" {
     var client: Client = undefined;
     initTestClient(&client, &pool);
 
-    var blocker: AdBlocker = try .init(testing.allocator);
+    var blocker: AdBlocker = try .init(testing.allocator, testing.test_app.regex_context);
     defer blocker.deinit();
     var list: std.Io.Reader = .fixed(
         \\||ads.example.com^