refactor: let updaters define the files they want to run on

This change reverses the responsibility for which files the updaters are
run on. Now each updater can specify the list of files and wether the
files should be created when they do not exist yet. This simplifies the
handling of each update in releaserpleaser.go, as we can just iterate
over all updaters and call it for each file of that updater.

Also update the flags to allow users to easily define which updaters
should run.

internal/updater/generic.go

@@ -7,8 +7,26 @@ import (
 
 var GenericUpdaterSemVerRegex = regexp.MustCompile(`\d+\.\d+\.\d+(-[\w.]+)?(.*x-releaser-pleaser-version)`)
 
-func Generic(info ReleaseInfo) Updater {
-	return func(content string, filename string) (string, error) {
+func Generic(files []string) Updater {
+	return generic{
+		files: files,
+	}
+}
+
+type generic struct {
+	files []string
+}
+
+func (g generic) Files() []string {
+	return g.files
+}
+
+func (g generic) CreateNewFiles() bool {
+	return false
+}
+
+func (g generic) Update(info ReleaseInfo) func(content string) (string, error) {
+	return func(content string) (string, error) {
 		// We strip the "v" prefix to avoid adding/removing it from the users input.
 		version := strings.TrimPrefix(info.Version, "v")