Use string instead of OptionValueSource

OptionValueSource is only meaningful as defined in
commander-js/extra-typings#3

typings/index.d.ts

@@ -272,7 +272,6 @@ export interface OutputConfiguration {
 
 export type AddHelpTextPosition = 'beforeAll' | 'before' | 'after' | 'afterAll';
 export type HookEvent = 'preSubcommand' | 'preAction' | 'postAction';
-export type OptionValueSource = 'default' | 'config' | 'env' | 'cli' | 'implied';
 
 export type OptionValues = Record<string, any>;
 
@@ -595,17 +594,17 @@ export class Command {
   /**
    * Store option value and where the value came from.
    */
-  setOptionValueWithSource(key: string, value: unknown, source: 'config'): this;
+  setOptionValueWithSource(key: string, value: unknown, source?: string | undefined): this;
 
   /**
    * Get source of option value.
    */
-  getOptionValueSource(key: string): OptionValueSource | undefined;
+  getOptionValueSource(key: string): string | undefined;
 
   /**
     * Get source of option value. See also .optsWithGlobals().
    */
-  getOptionValueSourceWithGlobals(key: string): OptionValueSource | undefined;
+  getOptionValueSourceWithGlobals(key: string): string | undefined;
 
   /**
    * Alter parsing of short flags with optional values.