editor/code: Enable TypeScript's `--exactOptionalPropertyTypes`

[tetsuharuohzeki]

This issue proposes to enable TS' --exactOptionalPropertyTypes option.

To enable this option,

• npm:@types/vscode chages their optional property types to accept undefined (e.g. bar?: Foo should be bar?: Foo | undefined if there is no problem).
• Or we need to change our code to avoid to assign undefined to properties as marked as optional.