Improve handling of boundless and unknown intervals

[cmoesel]

Add special case logic for boundless intervals (e.g., Interval[null, null]) and unknown intervals (e.g., Interval(null, null)).

When we process an Interval that has both boundaries null, we can't infer the point type since we have no value to infer it from. This caused problems in the existing logic that needs to know the point type for some calculations. To address this, I've added special-case logic for these edge cases since we often don't really need to know the point type to arrive at the correct answer.

I initially tried implementing this by using the point type of the other argument in each operation, but this turned out to be kind of messy and still was not perfect (in particular, it failed tests like Interval[null, null] overlaps Interval[null, null]).

My CQL Long PR introduces the approach of using the resultType from the ELM when we can't easily infer types from the data. We should extend that to also work in these cases, but since we want to get this fix out ASAP, I decided to implement this approach first. We need this approach regardless since ELM does not always carry the resultType.

NOTE: The CI that runs npm audit currently fails, but the reported audit issues are currently not resolvable, so there's not much we can do.

Fixes #364.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.19298% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.56%. Comparing base (5bc96b7) to head (fd13dd2).

Files with missing lines	Patch %	Lines
src/datatypes/interval.ts	77.19%	0 Missing and 13 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
- Coverage   87.58%   87.56%   -0.02%     
==========================================
  Files          52       52              
  Lines        4606     4658      +52     
  Branches     1297     1331      +34     
==========================================
+ Hits         4034     4079      +45     
+ Misses        359      354       -5     
- Partials      213      225      +12     

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