Support context-specific snippet definitions

[pokey]

We currently support specifying the scopes in which a particular snippet definition is active but this is not yet implemented. We would like to implement this.

Note that we cannot support this until we have #785.

• To make this useful, we will need to stop returning the first matching definition we find, and instead prefer snippets that have more specific contexts. Specifically, snippets that match a scope should have higher precedence than snippets that only match on language id, which should in turn have higher precedence than ones that have no language id.
• We need to think about how this interacts with user snippets overriding core / third-party snippets. Eg if user's snippet is less specific, should it still override a core snippet that is more specific? (probably not)