Lets use const just like everyone else

[mzedeler]

See #1888

[sergiodxa]

You can use const if you want on your own code.

[mzedeler]

React router is really complicated and has a lot of functionality that can be really surprising, so I read the source code quite frequently. Using const in the places where it makes sense would make it much easier to read. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const#:~:text=Many%20style%20guides%20(including%20MDN's,non%2Dprimitives%20that%20are%20mutated.