Allow CompletionOptions to provide additional data

Currently server can specify trigger characters through CompletionOptions. The following additional data would also be helpful in creating a more desired IntelliSense experience:

• commit characters - i.e. aside from Enter, what characters when typed should commit the completion item selected. This could be the same as trigger character, a subset, or something completely different.
• dismiss characters - i.e. aside from Escape, what characters when typed should dismiss the completion session? An example of this would be semicolon.

Another idea @LukaszMendakiewicz had was that some of this data can be overridden for a given completion session. For example, a server can define default values for this during initialization, but each completion request could return different sets of commit and dismiss characters.

Start by locating the CompletionOptions definition and the completion request and response structures in the protocol. Review how initialization-time options differ from per-completion data, then confirm the intended semantics for commit and dismiss characters. Done means the protocol can represent both character sets and any agreed session-specific overrides.