Add isopen and isready commands to server protocol

[jkrumbiegel]

isopen can be used to check that a file already has a worker opened for it, which can then be closed if desired. The isready command is just to have something for quarto to establish that the server is functioning at all, before sending further commands to it.

Fixes #39

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.66%. Comparing base (2c38cbd) to head (7a14e18).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   86.54%   86.66%   +0.11%     
==========================================
  Files           5        5              
  Lines         446      450       +4     
==========================================
+ Hits          386      390       +4     
  Misses         60       60              

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