prefer-web-first-assertion fails to detect the right variable declarator

[An631]

Whenever the expect call is used with an assigned variable, the code fails to find the appropriate variable declarator in the code.

For example:

        const locatorFoo = page.locator(".foo")
        const isBarVisible = await locatorFoo.locator(".bar").isVisible()
        expect(isBarVisible).toBe(false)

In the current code we are only checking for the first VariableDeclarator in the scope of the isBarVisible variable, this means that the code grabs the locatorFoo variable and verifies if it has a call to a non-web-first assertion like isVisible().

This causes very random and incorrect behaviors in the IDE red squiggles and in the suggested automated fixes.

I have a working change with tests to fix this already if I can get PR reviews for it.

[An631]

I have also identified and fixed (in same PR) another incorrect behavior in the rule.

We were ignoring all 'AssignmentExpressions' to mutable variables used inside the expect call expression.
For example:

let foo = 'foo';
foo = page.locator('.foo').textContent();
expect(foo).toBe('foo');