Add permanent header-only and compiled library targets with CMake export support

[grdanny]

• replaced the PYSTRING_HEADER_ONLY build option with two permanent targets: pystring::pystring (compiled) and pystring::pystring_header_only (header-only). Consumers pick whichever they need.
• added exported targets and package config files so downstream projects can consume pystring via find_package(pystring) and link against pystring::pystring or pystring::pystring_header_only.
• Adjusted CI - making two test executables - pystring_test and pystring_test_header_only run on every CI invocation, verifying both modes work correctly.
• Added test_header_only_define.cpp which fails the build with #error if PYSTRING_HEADER_ONLY isn't correctly propagated to consumers of the header-only target.
• revving up release number to 1.2.0 to indicate build system changes.

Assisted-by: Claude / Sonnet 4.6

[lgritz]

I guess? It looks ok, but I really don't feel like I have a full view of how this package is built and used to recognize if it were wrong.