This allows users to pick up the newly released version 2.3.0 of
exemplar.

These file extensions likely denote files that should not be checked
in.

This is flagged by markdownlint.

to enforce file.license_id from the beman standard

Add missing SPDX license identifiers to enforce file.license_id from the beman standard

This is needed to support modules.

Static libraries are currently incompatible with our modules design--
not inherently so, but I just haven't done the work yet to couple them
together, and deploying modules is higher priority (we have many
libraries that implement modules, but to my understanding, none of our
libraries need to have .cpp files currently).

This approach to modules implementation is based on the approach used
to implement module support for Boost by Rubén Pérez (anarthal). See
his documentation:
https://github.com/anarthal/boost-cmake/blob/e29bc2a5bd3493d353bf597610d4b32b5a11d0dc/modules.md

The basic idea is that every header is in one of three modes:

- Normal mode (the user included the header, and we aren't building
  with modules)
  - In this case, the macros configure the header so its contents are
    the same as their pre-refactor state
- included-from-interface-unit-mode
  - Here, we're inserting all the code from the header into an export
    block inside the implementation of the .cppm file, so this mode
    is the same as normal mode except that we ifdef out any standard
    library includes, or any other dependencies that the .cppm file
    is bringing in via module
- Compatibility header mode
  - The project is built as a module, but the consumer still is
    bringing the dependency in by writing #include <{...}.hpp>.
    In this case the header #ifdefs out everything except a simple
    import beman.foo statement.