Add a proposed API check option

Part of #2826

src/common/services/OptionsService.ts

@@ -41,6 +41,7 @@ export const DEFAULT_OPTIONS: ITerminalOptions = Object.freeze({
   macOptionClickForcesSelection: false,
   minimumContrastRatio: 1,
   disableStdin: false,
+  allowProposedApi: true,
   allowTransparency: false,
   tabStopWidth: 8,
   theme: {},

src/common/services/Services.ts

@@ -218,6 +218,7 @@ export interface IPartialTerminalOptions {
 }
 
 export interface ITerminalOptions {
+  allowProposedApi: boolean;
   allowTransparency: boolean;
   bellSound: string;
   bellStyle: 'none' | 'sound' /* | 'visual' | 'both' */;