feat: multi-language support — PHP, Ruby, Perl, Go (binary-env)

- PHP: ghcr.io/fission/php-env, entrypoint main.php::handler
- Ruby: ghcr.io/fission/ruby-env, entrypoint handler (single-word)
- Perl: ghcr.io/fission/perl-env (version=1), sub {} return value
- Go: ghcr.io/fission/binary-env with pre-compiled binary (shell script placeholder)

Provider changes:
- package_resource: added main.php, main.rb, main.pl to source_dir candidates
- function_resource: relaxed entrypoint validation to allow PHP (::), Ruby, Perl (single-word)
- function_resource_test: updated BadFormat test to reflect new valid formats

examples/php-hello/code/main.php

@@ -0,0 +1,7 @@
+<?php
+function handler($context)
+{
+    /** @var \Psr\Http\Message\ResponseInterface $response */
+    $response = $context["response"];
+    $response->getBody()->write("Hello from PHP in Fission");
+}