Assist: generate an is_empty method from a len method

[yoshuawuyts]

Sibling issue: #7708

Clippy's "len without is_empty" lint warns if you have a len method but no is_empty method. An assist could make this trivial to generate. I'd be happy to implement this; feel free to assign me to it!

Examples

before

impl MyStruct {
    p|ub fn len(&self) -> usize {
        self.data.len()
    }
}

after

impl MyStruct {
    pub fn len(&self) -> usize {
        self.data.len()
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }
}

[lnicola]

Implemented in #8037.