feat: make pixi-kernel work independently of kernel provisioner

[renan-r-santos]

This is especially important when trying to use it in VSCode where jupyter-client isn't used to launch a kernel.

[codecov]

Codecov Report

Attention: Patch coverage is 22.85714% with 54 lines in your changes are missing coverage. Please review.

Project coverage is 46.90%. Comparing base (b0e7972) to head (b87a759).

❗ Current head b87a759 differs from pull request most recent head 3d75d68. Consider uploading reports for the commit 3d75d68 to get more accurate results

Files	Patch %	Lines
src/pixi_kernel/__main__.py	0.00%	52 Missing ⚠️
src/pixi_kernel/pixi.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #18       +/-   ##
===========================================
- Coverage   59.75%   46.90%   -12.86%     
===========================================
  Files           3        3               
  Lines          82      113       +31     
  Branches       12       17        +5     
===========================================
+ Hits           49       53        +4     
- Misses         30       57       +27     
  Partials        3        3               

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.