Provide encoding-related APIs for editor extensions

[haoqunjiang]

Currently there are only two fields in the TextEditorOptions API, a few other TextEditor-related APIs, and none of them is able to deal with the text buffer's encoding.
Comparing to Atom's TextEdit API, that is far from enough.

Most importantly, the API limitation makes it (seems) impossible for vscode-editorconfig to implement features like charset support, which is a crucial need for many people.

[alexdima]

👍 Also trimTrailingWhitespaces is missing

[kmpm]

#751 and #844 would probably be helped by this as well.
At least #751 would be solvable if working with BOM were better and my issues would completely go away if there was a way to, per editor extension, set the encoding and other stuff.
So 👍

[jednano]

We also need access to the end of line sequence for EditorConfig.

[jrieken]

• set/get encoding
• set/get charset

[iHuahua]

T_T... my extension ......

[buzzzzer]

still backlog... :(

[vysker]

I also need this for my extension, as I explain here. Did not realize this issue already existed.

Definitely want this feature!