chore: include license text in crate artefact

[tommilligan]

Closes #86

Includes license texts in the crate archive - these were omitted by mistake.

Also:

• Updates the license identifier to a valid SPDX identifier; use of / instead of OR is deprecated by cargo: https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields.
• Prevent benchmarks from being accidentally published

[codecov-commenter]

Codecov Report

Merging #87 (092702f) into main (d063c77) will decrease coverage by 0.50%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   96.85%   96.35%   -0.51%     
==========================================
  Files           4        3       -1     
  Lines         191      192       +1     
==========================================
  Hits          185      185              
- Misses          6        7       +1     

Impacted Files	Coverage Δ
pretty_assertions/src/printer.rs	95.58% <0.00%> (-3.66%)	⬇️
...assertions_bench/benches/benchmarks/pretty_diff.rs
pretty_assertions/tests/macros.rs	98.00% <0.00%> (+0.04%)	⬆️

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