v0.7.8 Integration tests + bug fixes + go1.21 update

[janpfeifer]

• Added support for --raw_errros, where errors are not reported using HTML. Useful for
  running tests, for instance with nbmake (see Error handling #48, by @bagel897).
• Added functional/integration tests by instrumenting nbconvert (bringing coverage from ~25% to ~55%).
• Added run_coverage.sh to include integration tests in coverage report.
• Coverage configured to be generated manually (and not automatically in GitHub actions) -- coverage
  badge still generated in GitHub actions.
• Installation uses $JUPYTER_DATA_DIR, if it is set.
• Fixed proper shutdown.
• Fixed gopls dying when a cell is interrupted.
• Updated go.mod parser dependency for go 1.21 -- since the format changes slightly (see Cannot handle go 1.21.0 work files #53).

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This discussion was created from the release v0.7.8 Integration tests + bug fixes + go1.21 update.