Share unit definitions between Go and Javascript.

[ghemawat]

Go unit definitions (from measurement package) are now converted to JSON and delivered to Javascript code so that we do not need to duplicate these definitions in Javascript.

Had to make the unit type definitions exported with public fields.

Also had to change a test case to expect "us" instead of "µs".

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 66.92%. Comparing base (0ed6a68) to head (0b38b77).
Report is 14 commits behind head on main.

Files	Patch %	Lines
internal/measurement/measurement.go	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
+ Coverage   66.86%   66.92%   +0.05%     
==========================================
  Files          44       44              
  Lines        9824     9792      -32     
==========================================
- Hits         6569     6553      -16     
+ Misses       2794     2784      -10     
+ Partials      461      455       -6     

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[aalexand]

Thanks!