add missing keyword completion

[kkalb]

Closes #937

added true, false and nil as keywords for completion

Missing command key in resulting item map. Any suggestions?
Also I used TDD and did not manage to test this in vsc-elixir-ls or similar yet.

Update: I replaced the submodule of vscode-elixir-ls with my fork of elixir-ls and it worked; check screenshots:

Also I do not know which priority they deserve.

Update 2:

May we add a new kind atom at

case kind do
...
- :value -> 12
+ :atom-> 12
+ :value -> 13
...

or

case kind do
...
- :keyword -> 14
+ :atom-> 14
+ :keyword -> 15
...

or do we leave it as

:keyword

[lukaszsamson]

I merged it. Let's see how it works in practice