Fix file lookup in parser keywords; Refactor nu_repl

[kubouch]

Description

Fixes the logic used to look up files in parser keywords which was broke NU_LIB_DIRS.

In order to test it, I had to use the nu_repl() simulator. However, I discovered that nu_repl() changes directory for the whole test binary which would cause a lot of other tests fail. To address that, I moved nu_repl() to be a testbin which means it will run in its own Nushell instance. This means, it's possible to run nu --testbin=nu_repl line1 line2 and it would be executed as if you typed

> line1

> line2

in the Nushell REPL.

Fixes #6184

Tests

Make sure you've done the following:

• Add tests that cover your changes, either in the command examples, the crate/tests folder, or in the /tests folder.
• Try to think about corner cases and various ways how your changes could break. Cover them with tests.
• If adding tests is not possible, please document in the PR body a minimal example with steps on how to reproduce so one can verify your change works.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all the tests pass