refactor: prep for additional licenseHeader logic

this refactors licenseHeader to make it easier to add additional logic
to determine prefix, beyond just file extension.

main.go

@@ -277,12 +277,15 @@ func fileHasLicense(path string) (bool, error) {
 	return hasLicense(b) || isGenerated(b), nil
 }
 
+// licenseHeader populates the provided license template with data, and returns
+// it with the proper prefix for the file type specified by path. The file does
+// not need to actually exist, only its name is used to determine the prefix.
 func licenseHeader(path string, tmpl *template.Template, data licenseData) ([]byte, error) {
 	var lic []byte
 	var err error
-	switch fileExtension(path) {
-	default:
-		return nil, nil
+	base := strings.ToLower(filepath.Base(path))
+
+	switch fileExtension(base) {
 	case ".c", ".h", ".gv", ".java", ".scala", ".kt", ".kts":
 		lic, err = executeTemplate(tmpl, data, "/*", " * ", " */")
 	case ".js", ".mjs", ".cjs", ".jsx", ".tsx", ".css", ".scss", ".sass", ".tf", ".ts":
@@ -307,11 +310,13 @@ func licenseHeader(path string, tmpl *template.Template, data licenseData) ([]by
 	return lic, err
 }
 
+// fileExtension returns the file extension of name, or the full name if there
+// is no extension.
 func fileExtension(name string) string {
 	if v := filepath.Ext(name); v != "" {
-		return strings.ToLower(v)
+		return v
 	}
-	return strings.ToLower(filepath.Base(name))
+	return name
 }
 
 var head = []string{