fix(isFQDN): allow all-underscore labels with allow_underscores (#1253)

[gibson042]

Update isFQDN to allow domain names with all-underscore labels when allow_underscores is true. Underscore is not allowed in LDH "preferred name syntax" host names, but may appear anywhere in generic domain name labels (cf. RFC 8499).

Checklist

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

[codecov]

Codecov Report

Merging #1469 (6bb6b93) into master (a19ebff) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1469   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          96       96           
  Lines        1286     1286           
=======================================
  Hits         1285     1285           
  Misses          1        1           

Impacted Files	Coverage Δ
src/lib/isFQDN.js	100.00% <100.00%> (ø)

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[profnandaa]

LGTM. pls fix the merge conflict and we should be good to go. Thanks for your contrib! 🎉

[gibson042]

Done.