[Feature] Add support for a `[dev_dependencies]` table in `lux.toml`

[benniekiss]

It would be helpful to be able to define additional dependencies that are only installed in dev environments.

One instance where this would be helpful is if a project contains benchmarks which require external modules that would otherwise not be needed for the project.

[mrcjkb]

Commands that could make use of this:

• lx shell
• lx run
• lx lua
• ... any others?

[benniekiss]

Not that I can think of! lx lua was where I realized the need