Make $nu constant

[kubouch]

Description

This PR makes $nu a parse-time constant. The benefits are two-fold:

1. $nu is now usable in more situations. For example, reloading the config file is now more convenient than ever:

source $nu.config-path

2. Performance. Evaluating $nu is now approximately 15x faster:

> use std
> std bench { 1..1000 | each { $nu } } --pretty
# before:
80ms 681µs 622ns +/- 1ms 185µs 491ns
# after:
5ms 211µs 764ns +/- 1ms 39µs 16ns

The only remaining question is where to move the eval_nu_variable() since it is now not a part of the engine. I'm leaning towards nu-protocol which would move the sysinfo dependency from nu-engine to nu-protocol, alongside to introducing nu-path dependency.

User-Facing Changes

Enhances $nu usability, but everything should work exactly as before.

Tests + Formatting

After Submitting

[sophiajt]

Let's do it