LibWeb: Implement SubtleCrypto.digest()

This is a simple implementation of SubtleCrypto.digest() using LibCrypto
under the hood, so it supports all the required hash functions:
SHA-1, SHA-256, SHA-384, SHA-512.

Two FIXMEs remain: doing the hashing "in parallel", and supporting an
object argument instead of a plain string.

Userland/Libraries/LibWeb/Crypto/SubtleCrypto.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <LibJS/Forward.h>
 #include <LibWeb/Bindings/Wrappable.h>
 
 namespace Web::Crypto {
@@ -21,6 +22,8 @@ public:
         return adopt_ref(*new SubtleCrypto());
     }
 
+    JS::Promise* digest(String const& algorithm, JS::Handle<JS::Object> const& data);
+
 private:
     SubtleCrypto() = default;
 };