Fix/diagnostics panel crash

[krassowski]

References

Fixes #516

Code changes

• Added event listener to listen for closure of foreign documents and remove their diagnostics
• Added a safety net (if + console.warn) in case we get an empty document again for another reason
• Reduced default remaining_lifetime of foreign documents to 6. This is an arbitrary number but it feels right given that we are balancing the benefit of having faster restoration of features if foreign document temporarily stopped matching the regular expression as user is modifying it and the con of the diagnostics in the panel not disappearing immediately

User-facing changes

Diagnostics panel will work reliably when removing transclusions/foreign documents

Backwards-incompatible changes

None

Chores

• linted
• tested
• documented
• changelog entry