Allow specifying a topic when subscribing to a document

[humdrum]

What this PR does / why we need it:

• provide a new API that allows users to subscribe to the document with a specific target path.
• improve the event value of Local/RemoteChange.

Which issue(s) this PR fixes:

Fixes #74 #76

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

document.eventStream is removed. Use document.subscribe() to catch the events.
Please refer to the samples.

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #78 (b85832c) into main (8e84992) will increase coverage by 0.41%.
The diff coverage is 63.87%.

❗ Current head b85832c differs from pull request most recent head 48e762a. Consider uploading reports for the commit 48e762a to get more accurate results

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   55.54%   55.96%   +0.41%     
==========================================
  Files          92       92              
  Lines       14303    14649     +346     
==========================================
+ Hits         7945     8198     +253     
- Misses       6358     6451      +93     

Impacted Files	Coverage Δ
Sources/Document/DocEvent.swift	0.00% <0.00%> (ø)
Sources/Document/Json/JSONText.swift	73.57% <ø> (+17.78%)	⬆️
Tests/Integration/DocumentIntegrationTests.swift	0.45% <0.00%> (-0.16%)	⬇️
Sources/Document/Operation/Operation.swift	60.86% <58.13%> (-39.14%)	⬇️
Sources/Document/Operation/SelectOperation.swift	46.34% <75.00%> (-5.28%)	⬇️
Sources/Document/Document.swift	48.82% <79.26%> (+8.28%)	⬆️
Sources/Document/Operation/AddOperation.swift	92.30% <81.81%> (-4.25%)	⬇️
Sources/Document/Operation/IncreaseOperation.swift	73.80% <81.81%> (-7.45%)	⬇️
Sources/Document/Operation/EditOperation.swift	58.82% <85.71%> (-0.16%)	⬇️
Sources/Document/Operation/SetOperation.swift	70.27% <85.71%> (+2.52%)	⬆️
... and 10 more

... and 1 file with indirect coverage changes