fix: stop static Picker.Item from referencing class type parameters

[cpruijsen]

Type Item as typeof PickerItem with a generic PickerItem class, matching PickerIOS and static Item: typeof PickerItem = PickerItem in js/Picker.js. PickerProps<T> / onValueChange are unchanged.

Fixes #307
Also covers #339

Summary

static Item: React.ComponentType<PickerItemProps<T>> is invalid TypeScript (TS2302: static members cannot reference class type parameters). TypeScript documents this at https://www.typescriptlang.org/docs/handbook/2/classes.html#type-parameters-in-static-members

Same error as #216. #222 fixed it by using ItemValue; #302 (v1.16.5) put T back on Item and the error returned. Still present on current main. Consumers that leave skipLibCheck at the default (false) fail; this repo's tsc job did not catch it because tsconfig.json sets "skipLibCheck": true.

validate:typescript now also runs a check that typechecks typings/ with skipLibCheck: false and only asserts diagnostics in that folder, so this error cannot silently return.

The class is used instead of static Item: React.ComponentType<PickerItemProps<ItemValue>> (the #222 one-liner). The one-liner is enough to clear TS2302 and is what #216 confirmed. The class is the same Item-class pattern already in this repo, and Picker.Item stays generic (the intent of #302) without naming Picker's T on a static. Can switch to the one-liner if that is preferred.

Test plan

• yarn validate:typescript passes
• In an app without skipLibCheck: true, tsc no longer reports TS2302 from node_modules/@react-native-picker/picker/typings/Picker.d.ts
• <Picker.Item label="Java" value="java" /> still typechecks
• Picker selectedValue / onValueChange generics still infer