Add support of "c++20-pmr" c++ language flavor

[serges147]

• Needed for libcyphal project where I'm planning to use it like this:

    if (CMAKE_CXX_STANDARD STREQUAL "14")
        list(APPEND LOCAL_NNVG_CMD_ARGS "--language-standard=cetl++14-17")
    else ()
        list(APPEND LOCAL_NNVG_CMD_ARGS "--language-standard=c++${CMAKE_CXX_STANDARD}-pmr")
    endif ()

• cetl++14-17 now uses cetl::pf17::variant for union types.
• "pmr" language feature now propagates internal allocator_ field.
• Generation of a type members initialization a bit changed (stylistically) for easier generation (: now on a new line)

BTW, these changes are successfully in use at libcyphal PR: OpenCyphal-Garage/libcyphal#389
where I bring up registry list/access servers at Example_2_Application_0_NodeHeartbeatGetInfo_Udp .

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud