Language Server Index Format: use a non-UTF-16 encoding as the default for `positionEncoding`

From the lengthy discussions in https://github.com/microsoft/language-server-protocol/issues/376 , a lot of language servers and non-JavaScript based clients have been inconvenienced due to the UTF-16 position counting. In Language Server Index Format, which does not have the backward compatibility burden, we should probably move away from UTF-16 which is considered inferior by a majority of developers. As the recommended default, both UTF-32 code points and UTF-8 bytes can work but it seems that code points are convenient for most implementations.

When opening a non UTF-8 encoded file, an implementation may likely represent it with a sequence of code points. I believe most implementations have convenient APIs to count code points but counting UTF-8 bytes may require more transcoding.

  /**
   * The string encoding used to compute line and character values in
   * positions and ranges. Currently only 'utf-16' is support due to the
   * limitations in LSP.
   */
  positionEncoding: 'utf-16',

A survey on the Position.character offsets encoding supported by the clients:

Start by reviewing the linked LSP position-encoding discussion and the TypeScript definition for positionEncoding shown in the issue. Determine the agreed non-UTF-16 default and its compatibility implications; the work is done when the LSIF specification and related definition consistently document and use that default.