(fix) fixes to formatDate expression and adds tests for built-in expressions helpers

[pirupius]

Requirements

• This PR has a title that briefly describes the work done including the ticket number. If there is a ticket, make sure your PR title includes a conventional commit label. See existing PR titles for inspiration.
• My work conforms to the OpenMRS 3.0 Styleguide and design documentation.
• My work includes tests or is validated by existing tests.

Summary

• formatDate expression was actually parsing string to date so the logic has been moved to parseDate (this was introduced as part of AFE parity)
• formatDate now gives the option to format dates based on locale or a format string
• Adds custom expression tests and clean up of existing ones

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[github-actions]

Size Change: +30.1 kB (+2.63%)

Total Size: 1.18 MB

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dist/151.js	330 kB	+30.1 kB (+10.04%)	⚠️

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dist/main.js	342 kB	+42 B (+0.01%)
dist/openmrs-esm-form-engine-lib.js	3.67 kB	+1 B (+0.03%)

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