Support environments without rustup

[alexheretic]

This can be worked around currently by setting rlsCommandOverride to rls. However, we should support this without any tinkering.

• Fall back to system rls if rustup doesn't exist.
• Disable or remove rustup-y config if possible, otherwise show an error notification if the config is changed as it won't have an effect.

Related #81, #114

[cnd]

I need that, also it could be nice to have documentation for tinkering...