feat: add .type() unary method

[divarvel]

Add .type() method and associated tests.

note: there is a bit of a missed opportunity in not having the types part of the default symbol table, but:

1. modifying the default symbol table is a breaking change
2. the default symbol table is primarily for saving space on biscuits, while the type strings should mostly be used on authorizers (they can still happen in checks, but see reason 1.)

[codspeed-hq]

CodSpeed Performance Report

Merging #201 will not alter performance

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

Summary

✅ 12 untouched benchmarks

[codecov]

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 ⚠️
biscuit-auth/src/token/builder.rs	0.00%	1 Missing ⚠️
biscuit-parser/src/builder.rs	0.00%	1 Missing ⚠️

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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