find: accept egrep and posix-egrep regex types to match GNU

[AlejandroCoronadoN]

GNU find accepts egrep and posix-egrep as -regextype names; both select the extended (ERE) syntax. uutils rejects them:

$ find . -regextype egrep -regex '.*'
find: Invalid regex type: egrep (must be one of 'emacs', 'grep', 'posix-basic', 'posix-extended')

They behave identically to the already-supported posix-extended. Verified against GNU with a + quantifier: egrep, posix-egrep and posix-extended all match the same paths. This maps both aliases to PosixExtended, mirroring how ed/sed already alias posix-basic.

Left out on purpose: GNU's awk, gnu-awk, posix-awk and posix-minimal-basic have small semantic differences that the onig syntaxes do not represent exactly, so they are not added here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.29%. Comparing base (42ddfb1) to head (1df2e56).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #848   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files          35       35           
  Lines        7445     7452    +7     
  Branches      388      388           
=======================================
+ Hits         6871     6878    +7     
  Misses        433      433           
  Partials      141      141           

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[github-actions]

Commit 1df2e56 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 317 / PASSED: 274 / FAILED: 38 / SKIPPED: 5
  Reference: TOTAL: 317 / PASSED: 274 / FAILED: 37 / SKIPPED: 6

Changes from main branch:
  TOTAL: +0
  PASSED: +0
  FAILED: +1

New test failures (1):
  - gnu/regextype_egrep

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing AlejandroCoronadoN:regextype-egrep-aliases (1df2e56) with main (42ddfb1)}

[AlejandroCoronadoN]

Closing this. The bfs test gnu/regextype_egrep shows that GNU's egrep syntax is not equivalent to posix-extended: GNU accepts patterns like *.*/{l (a leading * and an incomplete { are treated as literals), while posix-extended (Oniguruma) rejects them with "target of repeat operator is not specified". So mapping egrep/posix-egrep onto PosixExtended is not correct parity. A proper fix needs an egrep-equivalent syntax, which is more involved. Sorry for the noise.