Start scoping logic

[kubouch]

This PR:

• Adds a scoping logic with symbol name -> node ID lookup resolution (currently only variables are supported as we don't parse other definitions yet).
• Scopes are entered / exited when parsing a block.
• Fixes variable declaration parsing such that let $x = 10 resolves to variable name x, not $x.
• The scoping logic accounts for the future addition of overlays, but they haven't been added yet.
• When parsing a variable ($name), find_variable() is used to check if the variable exists.

[kubouch]

I'll try to move the scoping into a separate pass.

[kubouch]

Closed in favor of https://github.com/sophiajt/new-nu-parser/pull/18