Feature request: Const declarations in methods and functions

[MikaelMayer]

Following the ideas of this video
https://youtu.be/f74PSgwLdDk?si=M5llP57gTIwiw3dA&t=969

We wish to have

method Test() {
  const x := 2;
  x := x + 1; // Error !
}

that could prevent accidental rewrite of a variable, like in JavaScript, or Rust (let without mut)

Similarly, in functions, we could benefit from this syntax and deprecate the "var" there, since variables are not overridable by default.

function Test(): int {
  const x := 2
  x
}

The semi-colon could be optional for functions.

[MikaelMayer]

Related to #5914