showInputBox (and potentially others) support markdown in user visible strings

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Specifically I wanted to add markdown to the validation message here: https://github.com/microsoft/vscode/blob/948711473e69ff3cca8201d7101277f95d2d4572/src/vscode-dts/vscode.d.ts#L1994 I think there are likely other spots where markdown would be useful as well. Seems like any 'string' displayed to a user would be nice to support markdown/html in it.