Add unary to parse

[github-actions[bot]]

Benchmark diff:

goos: linux
goarch: amd64
pkg: github.com/cloudflare/pint/cmd/pint
cpu: AMD EPYC 7763 64-Core Processor                
              │   old.txt   │              new.txt               │
              │   sec/op    │   sec/op     vs base               │
FindEntries-4   39.96m ± 4%   39.78m ± 4%       ~ (p=0.739 n=10)
CheckRules-4     8.494 ± 2%    8.539 ± 2%       ~ (p=0.280 n=10)
geomean         582.6m        582.8m       +0.03%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.353 n=10)
CheckRules-4    979.5Mi ± 0%   979.2Mi ± 0%       ~ (p=0.579 n=10)
geomean         163.8Mi        163.8Mi       -0.01%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.303 n=10)
CheckRules-4    14.21M ± 0%   14.21M ± 0%       ~ (p=0.796 n=10)
geomean         1.369M        1.369M       -0.01%

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.12%. Comparing base (2a5d28e) to head (56d40cc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1122 +/- ## ======================================= Coverage 95.12% 95.12% ======================================= Files 102 102 Lines 10195 10201 +6 ======================================= + Hits 9698 9704 +6 Misses 346 346 Partials 151 151 ```

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