feat(semantic-conventions): add 'EVENT_*' exports for event names

[trentm]

This came up because open-telemetry/opentelemetry-js-contrib#2941 will be using some semconv event names in its instrumentation. See https://github.com/open-telemetry/opentelemetry-js-contrib/blob/c035ed216bdded07211edda501e1fe28540c13da/packages/instrumentation-openai/src/instrumentation.ts#L80-L86

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (48538b4) to head (6441028).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5832   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         303      303           
  Lines        7943     7943           
  Branches     1607     1607           
=======================================
  Hits         7547     7547           
  Misses        396      396           

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