ISSUE #58 - Add POSIX setgid/setuid/sticky bit support

Fixes #58

Description

Here, I adjust the POSIX_LS_RE to account for s, S, t, and T in the "pex"-y positions, by adding them to the set of characters we scan for 9 of. Now, the following characters are included in the scan:

-rwxsStT, where previously only -rwxs were supported. Off-hand, it appears the s was an attempt to support setuid/setgid, but it did not cover the case where the x bit is not set, which renders as a S.

Because this regex did not previously validate for the correct ordering (i.e., refuting strange combos like rxw), I did not add them in this PR.
The pex parsing closure, which does some simple arithmetic to determine the "pex" value, needed some updates.

S and T indicate the presence of a setuid/setgid bit, or a sticky bit, but without x. (If x is set, then the S instead is typically reported as s.)

So, I added S and T as exceptions in addition to -. They will be treated the same. With this change, no information is recorded about the setuid/setgid bits, and I don't think the interpretation of them is of great importance at this time given they are interpreted locally by the operating system and they have different meanings.

Please select relevant options.

[x] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update

[x] My code follows the contribution guidelines of this project
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] My changes generate no new warnings
[x] I formatted the code with cargo fmt
[x] I linted my code using cargo clippy and reports no warnings
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have introduced no C-bindings
[x] I increased or maintained the code coverage for the project, compared to the previous commit

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