Add initial implementations for encoder handling and conversion logic

Introduce encoder modules for various codecs and formats (e.g., h264_nvenc, libx264, libmp3lame). Add `Convertor` logic to retrieve supported formats via FFmpeg utilities and manage encoders for audio, video, and image processing.

internal/ffmpeg/encoder/libx264/encoder.go

@@ -0,0 +1,56 @@
+package libx264
+
+import (
+	"errors"
+	"git.kor-elf.net/kor-elf/gui-for-ffmpeg/internal/ffmpeg/encoder"
+)
+
+var Presets = []string{
+	"ultrafast",
+	"superfast",
+	"veryfast",
+	"faster",
+	"fast",
+	"medium",
+	"slow",
+	"slower",
+	"veryslow",
+	"placebo",
+}
+
+func NewEncoder() encoder.EncoderContract {
+	parameters := map[string]encoder.ParameterContract{
+		"preset": newParameterPreset(),
+	}
+	getParams := func(parameters map[string]encoder.ParameterContract) []string {
+		params := []string{"-c:v", "libx264"}
+
+		if parameters["preset"] != nil && parameters["preset"].IsEnabled() {
+			params = append(params, "-preset", parameters["preset"].Get())
+		}
+
+		return params
+	}
+
+	return encoder.NewEncoder("libx264", parameters, getParams)
+}
+
+func NewData() encoder.EncoderDataContract {
+	title := "libx264"
+	formats := []string{"mp4"}
+	fileType := encoder.FileType(encoder.Video)
+	return encoder.NewData(title, formats, fileType, NewEncoder)
+}
+
+func newParameterPreset() encoder.ParameterContract {
+	setParameter := func(s string) (string, error) {
+		for _, value := range Presets {
+			if value == s {
+				return value, nil
+			}
+		}
+
+		return "", errors.New("preset not found")
+	}
+	return encoder.NewParameter("preset", false, "medium", setParameter)
+}