Optimize rgba func

[owjs3901]

• rgba to hex
• rgb to hex
• remove F of last char when length of char is 4
• remove FF of last string when length of char is 8

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
libs/css/src/constant.rs	100.00% <100.00%> (ø)
libs/css/src/lib.rs	98.38% <ø> (ø)
libs/css/src/optimize_value.rs	100.00% <100.00%> (ø)

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