fix: increase strength of provider types

[florian-lefebvre]

To make #287 and #288 diffs smaller and easier to review:

• Improves some types
• Update unifont methods to directly be returned, to allow infering types (useful for generics)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.91%. Comparing base (dc0d37a) to head (5a7626e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #290   +/-   ##
=======================================
  Coverage   98.90%   98.91%           
=======================================
  Files          11       11           
  Lines         458      460    +2     
  Branches      112      113    +1     
=======================================
+ Hits          453      455    +2     
  Misses          5        5           

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[danielroe]

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