[FEATURE] encoding-related APIs for extensions

[AhkunTa]

如果你的需求与问题相关，请在下面描述一下（Is your feature request related to a problem? Please describe.） like https://github.com/microsoft/vscode/issues/824

描述你预期的功能表现（Describe the solution you'd like）

• [ ] get/set encoding

描述你考虑过的替代方案（Describe alternatives you've considered）

补充信息（Additional context）