Update template discovery to make its order deterministic

[MathisHammel]

Template files are discovered on the system using os.walk, which does not guarantee deterministic order. This can result in a messy user experience, where templates with the same prefix name in a directory will not appear together in the UI.

This PR makes the order consistent by sorting templates by path in lexicographic order.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.28%. Comparing base (67f5ee3) to head (6326078).
⚠️ Report is 55 commits behind head on main.

Files with missing lines	Patch %	Lines
jupyterlab_templates/extension.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #289   +/-   ##
=======================================
  Coverage   64.28%   64.28%           
=======================================
  Files           6        6           
  Lines          98       98           
  Branches       14       11    -3     
=======================================
  Hits           63       63           
  Misses         32       32           
  Partials        3        3           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[timkpaine]

de-minimis change for dco purposes, thanks for the change!