Implement `textDocument/completion` request

[Tobiky]

Protocol definition

Basic code completion. Should recommend symbols semantically available in current scope.

[tagyieh]

@Tobiky quick question about this issue. Suppose a user is typing a variable and wants to reference attack steps/associations belonging to an asset defined in an included file. Currently, nothing would be recommended, as we are only querying for things in the current file. Should we make this possible?

[Tobiky]

@Tobiky quick question about this issue. Suppose a user is typing a variable and wants to reference attack steps/associations belonging to an asset defined in an included file. Currently, nothing would be recommended, as we are only querying for things in the current file. Should we make this possible?

Excellent observation, though I think we'll leave that for v0.3.0 ( #48 ) instead since that one will handle other refinements of completion.