[prefer-readonly-signals] New rule for checking that signals are defined as readonly

[m-thalmann]

Description and reproduction of the issue

The new Angular signals should only be updated using the .set() or .update() function, never overwritten.
For this reason I propose a new rule which checks that all signals are marked as readonly.

Incorrect examples

myCounter = signal(0);
anotherCounter = input(0);
doubleCounter = computed(() => this.myCounter() * 2);

Correct examples

readonly myCounter = signal(0);
// ...