expr: Fix parsing regex anchors '^' and '$'

[frendsick]

This PR handles and tests the cases where '^' or '$' should be interpreted as special characters.

^ acts as a special character only at the beginning of the regular expression or subexpression (that is, after ( or |).

It ($) is the same as ^, but refers to end of pattern space. $ also acts as a special character only at the end of the regular expression or subexpression (that is, before ) or |)

Reference: https://www.gnu.org/software/sed/manual/html_node/BRE-syntax.html

fixes #7951, fixes #7952

[frendsick]

The Style/lint tests are failing, but they seem unrelated to expr and thus unrelated to this PR. How should I handle those, or do we leave them as is for now?

[frendsick]

The clippy errors are already reported by #7945

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)