Align capitalize with the built-in Capitalize<T> TypeScript type

[aleclarson]

As I mentioned on @rafawendel's Radash PR, the current capitalize differs in behavior from TypeScript's Capitalize<T> built-in type.

The built-in Capitalize type does not match the behavior of Radash capitalize.

type Foo = Capitalize<'foo Bar'>
//   ^? 'Foo Bar'

let foo = _.capitalize('foo Bar')
// => 'Foo bar'

I suggest we change Radashi's capitalize to align with it.