Add a boolean property to `TextDocument` if the encoding was guessed

[Colengms]

This is a request for API access to the original encoding of a file. i.e.:

(If this is already possible, please let me know. I couldn't find it.)

If I understand correctly, VS Code represents the content of open files to extensions as UTF-8. In the C/C++ Extension, there are scenarios in which IntelliSense should reflect how the compiler would interpret the file, which may depend on the files actual encoding. i.e. MSVC and string literal contents.

[Colengms]

In addition to the original encoding, it would also be very helpful to have access to (a bool indicating) whether or not the file was re-encoded from the encoding specified by the files.encoding setting (meaning that the file encoding was not able to be auto-detected). The C/C++ Extension indexes files in the workspace, honoring the files.encoding setting when reading directly from disk (if unable to detect the encoding). When that setting changes we should reparse files, preferably only the ones impacted by that change. We can determine ourselves for files read directly from disk. However, if open in the editor, we use the file buffer provided by VS Code. A change to files.encoding could indicate that the buffer we had used (from VS Code) may not have been translated correctly and should be reparsed.

[bpasero]

Related: #824

[bpasero]

I would like to go ahead and merge into #824 as per the original request. #209503 (comment) seems unrelated/additive, so I would suggest to report as individual new issue.