scanner: discard Form instances with no classified inputs

[sjbarag]

Reviewer: @GioSensation (while @shakyShane is unavailable)
Asana: https://app.asana.com/0/1206682621538333/1207230997693280/f

Description

When encountering <form/>s with no fillable inputs, the Scanner and Form classes previously maintained MutationObservers for each <form/>. This allowed the global MutationObserver to detect newly-added <input/>s and attribute them to existing Form instances without having to create a new Form, but also meant we maintained observers and listeners that often went unused. For performance reasons, we stop processing <form/> elements once 30 Form instances have been created. This allowed a large number of unfillable <form/>s could consume the entire Form pool before an actually-fillable <form/> is encountered.

Immediately destroy() a Form instance that contains no fillable <input/>s, and avoid adding that instance to a Scanner's set of known Form instances. This ensures form-scoped listeners and observers are only maintained when a fillable field is present. When a new <input/> is added the global MutationObserver and Scanner create a new Form instance to manage it, enabling slightly lazier listener/observer initialization.

Steps to test

1. npm run test
   • Note the change in submit button statistics:

     Category	Count	Percent
     False Positive: Before	638	47%
     False Positive: After	596	45%
     False Negative: Before	389	14%
     False Negative: After	383	15%

2. With these changes applied in either the browser extension or DDG browser, load https://www.scanmalta.com/shop (a page with currently a lot of <form/> elements).
3. Click "Hello, sign in" at the top (on a desktop-shaped browser) and ensure the email/password inputs are properly identified with data-ddg-inputtype="credentials.username" and data-ddg-inputtype="credentials.password.current" attributes

[dbajpeyi]

Looking very good to me, and tests are good too. Tested locally and the change works too! Merging it.