add commented out mold linker usage

[fdncred]

Description

This PR adds some settings that are commented out to the .cargo/config.toml. When one of these settings are uncommented, it will allow you to use the mold linker on linux and Mac. I add this here because whenever I want to use or test out mold, I always have to look up how to do it. Hopefully this solves that problem.

Major Changes

If you're considering making any major change to nushell, before starting work on it, seek feedback from regular contributors and get approval for the idea from the core team either on Discord or GitHub issue.
Making sure we're all on board with the change saves everybody's time.
Thanks!

Tests + Formatting

Make sure you've done the following, if applicable:

• 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 those in the tests

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 tests pass

After Submitting

• Help us keep the docs up to date: If your PR affects the user experience of Nushell (adding/removing a command, changing an input/output type, etc.), make sure the changes are reflected in the documentation (https://github.com/nushell/nushell.github.io) after the PR is merged.