- added doxygen page for the configuration options
- added SPDX-License-Identifiers around the place
- changed TOML_UNRELEASED_FEATURES default to 0
- simplified ABI namespace machinery
- fixed a number of doxygen bugs
also:
- fixed parser not handling floats with leading '.' characters
- added `TOML_PARSER` configuration option
- added `TOML_LIB_SINGLE_HEADER` indicator
- moved parse_error and the utf8 stream machinery to separate headers
- updated catch2