feat: add support for structured logging with the frankenphp_log() PHP function (#1979)

As discussed in https://github.com/php/frankenphp/discussions/1961,
there is no real way to pass a severity/level to any log handler offered
by PHP that would make it to the FrankenPHP layer. This new function
allows applications embedding FrankenPHP to integrate PHP logging into
the application itself, thus offering a more streamlined experience.

---------

Co-authored-by: Quentin Burgess <qutn.burgess@gmail.com>
Co-authored-by: Kévin Dunglas <kevin@dunglas.fr>

types_test.go

@@ -1,7 +1,6 @@
 package frankenphp
 
 import (
-	"io"
 	"log/slog"
 	"testing"
 
@@ -13,7 +12,8 @@ import (
 // this is necessary if tests make use of PHP's internal allocation
 func testOnDummyPHPThread(t *testing.T, test func()) {
 	t.Helper()
-	globalLogger = slog.New(slog.NewTextHandler(io.Discard, nil))
+
+	globalLogger = slog.Default()
 	_, err := initPHPThreads(1, 1, nil) // boot 1 thread
 	assert.NoError(t, err)
 	handler := convertToTaskThread(phpThreads[0])