Improve error message for name shadowing

[pfgithub]

Currently, the same error message is used for declaring a variable twice in the same scope and shadowing a variable from a parent scope

const X = 0;
const X = 0; // redefinition of 'X'

const Y = 0;
{
    const Y = 0; // redefinition of 'Y'
}

const Z = 0;
const Demo = struct {
    const Z = 0; // redefinition of 'Z'
};

This leads to confusion such as #8522 because the error, especially in structs, makes it sound like the declaration in the inner namespace has somehow ended up in the top level namespace instead of where it actually is.

This could be changed to something like name '…' shadows name from outer scope or something similar to make it clear that the declaration is in the right place and the issue is with name shadowing.

const X = 0;
const X = 0; // redefinition of 'X'

const Y = 0;
{
    const Y = 0; // name 'Y' shadows name from outer scope
}

const Z = 0;
const Demo = struct {
    const Z = 0; // name 'Z' shadows name from outer scope
};