pgsql: Fix conversion of SIMILAR TO regexes for character classes

Fix conversion of SIMILAR TO regexes for character classes

The code that translates SIMILAR TO pattern matching expressions to
POSIX-style regular expressions did not consider that square brackets
can be nested. For example, in an expression like [[:alpha:]%_], the
logic replaced the placeholders '_' and '%' but it should not.

This commit fixes the conversion logic by tracking the nesting level of
square brackets marking character class areas, while considering that
in expressions like []] or [^]] the first closing square bracket is a
regular character. Multiple tests are added to show how the conversions
should or should not apply applied while in a character class area, with
specific cases added for all the characters converted outside character
classes like an opening parenthesis '(', dollar sign '$', etc.

Author: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/16ab039d1af455652bdf4173402ddda145f2c73b.camel@cybertec.at
Backpatch-through: 13

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1fe15d25e65c5ef4fe3be9efd2927f3e3891f7b1

Modified Files
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src/backend/utils/adt/regexp.c | 38 +++++++++++++++++----
src/test/regress/expected/strings.out | 62 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/strings.sql | 20 +++++++++++
3 files changed, 114 insertions(+), 6 deletions(-)