feat: inline var in function body when possible

[unbyte]

This PR inlines var declarations like how let/consts are processed.

It's to be applied only when:

1. the var declaration belongs to a function body, and
2. the function is an arrow function, or
3. the arguments is shadowed

This PR significantly reduced the bundle size of our internal large project, because our target is es5 and there are many code structures like prop: function() { var x = value; return x; }

Example

Input

function test() {
  var x = 1;
  return x;
}

Current Output

function test() {
  var x = 1;
  return x;
}

New Output

function test() {
  return 1;
}