Migrate from v4 to v5

[filiptammergard]

In v5, @typescript-eslint/explicit-function-return-type was tuned on with option allowExpressions: true.

This means all functions need explicit return types:

- const Component = ():  => { ... }
+ const Component = (): JSX.Element => { ... }

- const double = (num: number) => num * 2
+ const double = (num: number): number => num * 2 

Expressions like this don't need explicit return types:

const foo = arr.map((i) => i * i) // this is still fine!