feat: `enabled` for sources

[ssiyad]

This PR adds enabled for individual sources.

This option can be used to toggle source based on context/conditions (https://github.com/hrsh7th/nvim-cmp/issues/806). I wanted dictionary to be enabled only in comments and, I was able to achieve that by

enabled = function (ctx)
    return context.in_treesitter_capture('comment')
end

[MariaSolOs]

Is there anything blocking this PR?

[PeterCardenas]

@hrsh7th anyway we can get this PR through?