Allow shadowing of same values ignored by no-unused-vars

[glebec]

Sometimes when you are forced to initialize a variable (e.g. object destructuring to get the "rest" of the props) you want to ignore that variable. Using _ as the variable name is a convention taken from other languages (where it is sometimes more than just a convention). However, _ variables can trigger the no-shadow rule if you have nested uses.

We should upgrade our no-shadow to use the allow prop as explained at https://eslint.org/docs/rules/no-shadow#allow. We should allow any variables that are ignored under no-unused-vars.