Provide version-aware schema for completion of "core" blocks

[radeksimko]

Closes #121
Closes #122
Closes #123
Closes #36

I have created two issues for the remaining work related to core schema (or at least what's left from the above issues):
#285 (backends)
#286 (provisioners)
and there is also an issue tracking modules:
#93 - i.e. we'll need to also find a way of exposing and reusing variables & outputs across modules, this PR just brings schema/completion for the static stuff (e.g. source or version).

[ghost]

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