Support for Message Operator

[cmoesel]

This PR adds support for user-provided MessageListeners that are invoked when a CQL Message operator should log a message. For more info see the spec for Message and the new documentation in Overview.md.

Submitter:

• [x] This pull request describes why these changes were made
• [x] Code diff has been done and been reviewed (it does not contain: additional white space, not applicable code changes, debug statements, etc.)
• [x] Tests are included and test edge cases
• [x] Tests have been run locally and pass
• [x] Code coverage has not gone down and all code touched or added is covered.
• [x] Code passes lint and prettier (hint: use yarn run test:plus to run tests, lint, and prettier)
• [x] All dependent libraries are appropriately updated or have a corresponding PR related to this change
• [x] cql4browsers.js built with yarn run build:browserify if source changed.

Reviewer:

Name:

• [x] Code is maintainable and reusable, reuses existing code and infrastructure where appropriate, and accomplishes the task’s purpose
• [x] The tests appropriately test the new code, including edge cases
• [x] You have tried to break the code

[codecov-commenter]

Codecov Report

Merging #247 (afcd7e5) into master (558adb8) will increase coverage by 0.05%. The diff coverage is 97.77%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   92.29%   92.34%   +0.05%     
==========================================
  Files          47       49       +2     
  Lines        4038     4078      +40     
==========================================
+ Hits         3727     3766      +39     
- Misses        311      312       +1     

Impacted Files	Coverage Δ
src/runtime/context.js	88.33% <90.90%> (-0.18%)	:arrow_down:
src/cql.js	100.00% <100.00%> (ø)
src/elm/expressions.js	100.00% <100.00%> (ø)
src/elm/message.js	100.00% <100.00%> (ø)
src/runtime/executor.js	72.72% <100.00%> (+2.72%)	:arrow_up:
src/runtime/messageListeners.js	100.00% <100.00%> (ø)

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