support more capabilities

Ref #31

[x] supportsConditionalBreakpoints
[x] supportsHitConditionalBreakpoints
[x] supportsSetVariable
[x] supportTerminateDebuggee
[ ] supportsTerminateRequest

This does not need to be supported. osv is attached, thus it only needs to support disconnect requests (ref: "Debug session end" on https://microsoft.github.io/debug-adapter-protocol/overview). We will still support supportTerminateDebuggee to terminate osv from the client.

jbyuki / one-small-step-for-vimkind