Add --delete to set-default-session, fix --session

[garrettjstevens]

This PR is a couple of small changes from when I was trying to figure out #1873.

• Fixes jbrowse set-default-session --session session.json so that it expects a session JSON file. It was previously expecting a config JSON file. Also added support for the session being in a top-level "session" attribute on the session JSON so that files created with File -> Export session can be used with set-default-session
• Adds a --delete option to set-default-session that removes the "defaultSession" from the config. Setting the default session to null or {} using set-default-session --session file.json caused errors, so this adds an easy way to clear the default session.

[codecov]

Codecov Report

Merging #1878 (64b0c75) into main (96256e1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1878   +/-   ##
=======================================
  Coverage   58.52%   58.52%           
=======================================
  Files         460      460           
  Lines       21288    21294    +6     
  Branches     5059     5061    +2     
=======================================
+ Hits        12459    12463    +4     
- Misses       8524     8526    +2     
  Partials      305      305           

Impacted Files	Coverage Δ
...ts/jbrowse-cli/src/commands/set-default-session.ts	96.36% <100.00%> (+0.44%)	⬆️
...r-view/src/BaseChordDisplay/components/Loading.tsx	95.00% <0.00%> (-5.00%)	⬇️
...inearGenomeView/components/RefNameAutocomplete.tsx	89.36% <0.00%> (-4.26%)	⬇️
packages/core/util/index.ts	81.28% <0.00%> (ø)
...ments/src/SNPCoverageAdapter/SNPCoverageAdapter.ts	95.00% <0.00%> (+0.83%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 96256e1...64b0c75. Read the comment docs.