Use stderr for tracing output

[filiptibell]

Rokit currently uses stdout for tracing output - this includes error messages such as when Rokit fails to run a tool.

This PR sets the tracing subscriber to use stderr, under the assumption that it is only used for warnings and hard errors, not regular output. Searching for the info! macro shows no matches so this should hold true.

Fixes #17

[filiptibell]

Turns out that this is actually more important than I originally thought - I had this happen:

1. Install StyLua in a specific project
2. Set the VSCode extension to try to use the Rokit-installed binary
3. Switch to a different project that does not use StyLua
4. Save a file after editing, it then formats on save, and the entire file gets wiped (!!) and replaced with an error message from Rokit saying StyLua does not exist for the project

I'll be merging this PR and making a new Rokit release with the fix.