style: typehint consts

[jkowalleck]

since php83 it is possible to static type class consts.
this is usefull when inheriting classes, and preventing wrong types on override that change the const ... downstream

no need to type consts in final classes

since this lib supports php<83, we cannot static type consts.
but we could add type annotations :D

[codacy-production]

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Coverage variation	Diff coverage
✅ +0.00%	✅ ∅ (target: 80.00%)

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	Coverable lines	Covered lines	Coverage
Common ancestor commit (5e66545)	5214	5201	99.75%
Head commit (a19dd3d)	5214 (+0)	5201 (+0)	99.75% (+0.00%)

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	Coverable lines	Covered lines	Diff coverage
Pull request (#434)	0	0	∅ (not applicable)

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