Only parse all-lowercase patterns as potential TF properties

[guineveresaenger]

This pull request asserts that according to Terraform best practices we should not parse backticked values containing uppercase letters as potential schema properties.

While it is a convention to use snake_case for TF properties, it is a strong one. The AWS provider requires it, and other providers follow this convention strictly as well.

The reason I would like to bring this change is because it allows us to detect documentation sections that are not actually nested property types, but rather lists of possible values, as described in #1507 in pulumi-mongodbatlas. Rather than add an ever-elusive regex for "Possible values include" et al, we can rely with reasonable confidence on the fact that TF properties are in general lowercase. See sample comparison diffs below.

A comparison diff for mongodbatlas shows multiple improvements. There are a few regressions but the main one is for a resource whose upstream doc is so unconventional that our representation of it is already flawed. Overall, we gain a lot more information than we're losing - the cost is that in a couple of places we get a nonsensical appendage of a real TF description. This should be fixed with docs overrides or by opening an upstream PR.
A comparison diff for AWS shows even more improvements, with only V2modelsBotVersionLocaleSpecification having a single regression, because the upstream docs are erroneously using camelCase.
A comparison diff for GCP shows only improvements.
A comparison diff for Azure shows only improvements.

Note I refactored the tests slightly because I wanted to be able to run individual tests and leave hints as to what they were testing.

Fixes #1507.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.56%. Comparing base (165111b) to head (1d76e74).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1969      +/-   ##
==========================================
+ Coverage   60.55%   60.56%   +0.01%     
==========================================
  Files         331      331              
  Lines       44722    44726       +4     
==========================================
+ Hits        27081    27090       +9     
+ Misses      16118    16114       -4     
+ Partials     1523     1522       -1     

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[iwahbe]

LGTM