feat: add `.type()` unary method

[divarvel]

Add .type() method and associated tests.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #201 will not alter performance

_{Comparing typeof (7c43662) with v5 (5792345)}

Summary

✅ 12 untouched benchmarks

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.96774% with 9 lines in your changes missing coverage. Please review.

Project coverage is 68.68%. Comparing base (5792345) to head (7c43662). Report is 2 commits behind head on v5.

Files with missing lines	Patch %	Lines
biscuit-auth/src/datalog/expression.rs	46.15%	7 Missing :warning:
biscuit-auth/src/token/builder.rs	0.00%	1 Missing :warning:
biscuit-parser/src/builder.rs	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##               v5     #201      +/-   ##
==========================================
+ Coverage   68.50%   68.68%   +0.17%     
==========================================
  Files          25       25              
  Lines        5626     5655      +29     
==========================================
+ Hits         3854     3884      +30     
+ Misses       1772     1771       -1     

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