fix: Cover rules including imported payees.

[bvanelli]

Implements the imported_payee type that was forgotten because it implements the exact same behaviour as string. There must be a reason why this was implemented, but as of now it implements exacly what STRING implements:

https://github.com/actualbudget/actual/blob/bbff543768eda550569c6e0a0a318446fbafaa0e/packages/loot-core/src/shared/rules.ts#L28-L50

Also trimmed the imported_payee as the original API does, and set it to the payee if it's missing.

Closes #67

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.06%. Comparing base (fb81211) to head (41c7c37).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
+ Coverage   97.05%   97.06%   +0.01%     
==========================================
  Files          14       14              
  Lines        1904     1911       +7     
==========================================
+ Hits         1848     1855       +7     
  Misses         56       56              

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