CMake improvements

[tcbrindle]

• Rename the top-level project to "flux" rather than "libflux"
• Rename the test executable to "test-flux" rather than "test-libflux"
• Add a flux-internal interface library target which we use to set compile options (mostly warning settings) which will be re-used between the tests and examples
• Make the tests and examples use the flux-internal library rather than the flux target itself
• Add various extra conformance flags for MSVC
• Enable various warnings for Clang
• Fix various MSVC warnings in the examples
• Use CMake's new target_sources(FILE_SET) command rather than the old-school target_include_directories
• Bump the required CMake version to 3.23 as a result (which is pretty new, but still older than our oldest supported compiler)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a45d844) 97.98% compared to head (f30651b) 97.98%.

:exclamation: Current head f30651b differs from pull request most recent head b03d276. Consider uploading reports for the commit b03d276 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #146 +/- ## ======================================= Coverage 97.98% 97.98% ======================================= Files 66 66 Lines 2383 2383 ======================================= Hits 2335 2335 Misses 48 48 ```

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