Refactor isTaxID for supporting more locales

[dspinellis]

• Prefix US-related functionality with "enUs"
• Remove assumption of global IRS-like campus prefixes
• Move US prefix check into separate function
• Add per-locale check function lookup table
• Support future locales lacking a check function (e.g. UK)
• Remove unneeded sorting of the IRS campus prefixes
• Add comments

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1409 into master will decrease coverage by 0.07%.
The diff coverage is 88.88%.

@@             Coverage Diff             @@
##            master    #1409      +/-   ##
===========================================
- Coverage   100.00%   99.92%   -0.08%     
===========================================
  Files           95       95              
  Lines         1248     1251       +3     
===========================================
+ Hits          1248     1250       +2     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
src/lib/isTaxID.js	94.73% <88.88%> (-5.27%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ed86b0a...2368cb4. Read the comment docs.

[dspinellis]

Code coverage will increase once another locale is supported (and tested). However, I think that this should be a separate commit.

[profnandaa]

LGTM, could you add the tests still as part of this PR?

/cc. @tux-tn @ezkemboi pls review.

[dspinellis]

For more tests to be added I'll need to add support for another locale. I don't think that such a change should be part of this PR.

One of my students has completed support for many more locales (including tests), and is ready to issue a PR once this gets accepted.

[ezkemboi]

@dspinellis what about we just add tests for the one locale/s currently supported/written here.
Leaving tests for code already written sometimes brings us back to add them, which could have been easy to make it inclusive of code.
As part of encouraging the students to see the best practice, I would suggest we add a test for this one as we wait for the other locales and its tests.
Also, tests give us surety of what we should expect to pass and what should fail. Without tests, it will not be easy for me/other developers to review the work as we will be forced to run against our own tests.
Once the test is added, I would love to review it.

[dspinellis]

The locale currently supported is already unit-tested; I wouldn't commit code without unit tests.

[dspinellis]

Here is the corresponding test.