Return Error from ParseLevel

[dezren39]

resolves #79

this adds an error result to the parse level function.

this change changes the return signature for ParseLevel!

users of ParseLevel will need to handle the error or choose to disregard it.

people will see the 'multiple values in a single context' error like below

log on  strict-parse-level [!?] via 🐹 v1.21.3 
  ❯ go test 
# github.com/charmbracelet/log [github.com/charmbracelet/log.test]
.\level_test.go:58:33: multiple-value ParseLevel(tc.level) (value of type (Level, error)) in single-value context
FAIL    github.com/charmbracelet/log [build failed]

[aymanbagabas]

Thank you @dezren39 for this work, a few comments below

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e5a733b) 77.63% compared to head (820a435) 80.25%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   77.63%   80.25%   +2.61%     
==========================================
  Files          10       10              
  Lines         635      699      +64     
==========================================
+ Hits          493      561      +68     
+ Misses        128      125       -3     
+ Partials       14       13       -1     

Files	Coverage Δ
level.go	92.85% <100.00%> (ø)

... and 4 files with indirect coverage changes

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