chore(lib): add debug feature

[jeertmans]

This adds a debug feature, similar to what Clap does, as requested by #379.

The current implementation only debugs a few parts of the code, and it may be interesting to add more debug messages in the different crates.

[github-actions]

Benchmark results:

group                                         before                                 changes
-----                                         ------                                 -------
count_ok/identifiers                          1.00   868.1±15.25ns   855.8 MB/sec    1.00    869.9±6.90ns   854.0 MB/sec
count_ok/keywords_operators_and_punctators    1.01      2.6±0.05µs   769.4 MB/sec    1.00      2.6±0.04µs   778.7 MB/sec
count_ok/strings                              1.04    597.4±7.97ns  1390.5 MB/sec    1.00   572.6±29.93ns  1450.7 MB/sec
iterate/identifiers                           1.00   883.5±38.52ns   840.8 MB/sec    1.01   891.2±18.00ns   833.6 MB/sec
iterate/keywords_operators_and_punctators     1.01      2.7±0.02µs   761.9 MB/sec    1.00      2.7±0.07µs   766.0 MB/sec
iterate/strings                               1.00   538.7±10.77ns  1542.1 MB/sec    1.04   562.3±28.98ns  1477.1 MB/sec