remove async call of subscribe callbacks

[humdrum]

What this PR does / why we need it:
DocEvent should be emitted synchronously with applying changes.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

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

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 41.84%. Comparing base (6cf3cb8) to head (e2d3c03).

Files	Patch %	Lines
Sources/Document/Document.swift	33.33%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #156   +/-   ##
=======================================
  Coverage   41.83%   41.84%           
=======================================
  Files         104      104           
  Lines       24681    24669   -12     
=======================================
- Hits        10326    10322    -4     
+ Misses      14355    14347    -8     

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