feat(messages): added custom media type

Author: sashokbg

javascript/src/messages.ts

@@ -471,7 +471,7 @@ export class Source {
 
   data: string = ''
 
-  mediaType: SourceMediaType = SourceMediaType.TEXT_X_CUCUMBER_GHERKIN_PLAIN
+  mediaType: SourceMediaType | CustomMediaType = 'text/x.cucumber.gherkin+plain'
 }
 
 export class SourceReference {
@@ -677,10 +677,9 @@ export enum PickleStepType {
   OUTCOME = 'Outcome',
 }
 
-export enum SourceMediaType {
-  TEXT_X_CUCUMBER_GHERKIN_PLAIN = 'text/x.cucumber.gherkin+plain',
-  TEXT_X_CUCUMBER_GHERKIN_MARKDOWN = 'text/x.cucumber.gherkin+markdown',
-}
+export type BaseMediaType = 'text/x.cucumber.gherkin';
+export type CustomMediaType = `${BaseMediaType}+${string}`;
+export type SourceMediaType = 'text/x.cucumber.gherkin+plain' | 'text/x.cucumber.gherkin+markdown';
 
 export enum StepDefinitionPatternType {
   CUCUMBER_EXPRESSION = 'CUCUMBER_EXPRESSION',