Implement LHS/RHS of assignment operator

[romgrk]

Like this but with treesitter support.

[theHamsta]

I guess this either requires: https://github.com/nvim-treesitter/nvim-treesitter/pull/493 or we just have a assignment textobject whose "inner" is either rhs or lhs.

For python:

(assignment
 left: (_) @lhs.inner
 right: (_) @rhs.inner)

or

(assignment
 left: (_) @assignment.inner
 right: (_) @assignment.inner) @assignment.outer

[kiyoon]

Closing as completed in #386