Fix toString for Date/DateTime

[cmoesel]

The minimum Date was incorrectly toStringing to 1-01-01 instead of 0001-01-01. This fixes the string formatting for this use case and applies more consistent padding to date-based toString operations across the board.

Pull requests into cql-execution require the following.
Submitter and reviewer should ✔ when done.
For items that are not-applicable, mark "N/A" and ✔.

Submitter:

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

Reviewer:

Name: Elsa Perelli

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.58%. Comparing base (041ed96) to head (c9b0f52).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
- Coverage   87.58%   87.58%   -0.01%     
==========================================
  Files          52       52              
  Lines        4607     4606       -1     
  Branches     1297     1297              
==========================================
- Hits         4035     4034       -1     
  Misses        359      359              
  Partials      213      213              

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[elsaperelli]

Lgtm!