Fixes for Orion Browser #3610

[tyirvine]

Description

This pull request has some reference to issue #3610. But mainly this pull request is so we can run Vimium in Orion. For those that don't know, Orion is a WebKit based browser that is attempting to support both Chrome and Firefox extensions alike.

I believe Vimium was working previously for the most part but then something to do with settings parsing broke it I believe. The issue I found was that WebKit's support for JSON.parse() seems to have a few issues.

1. There was a ^ symbol in vimium.css that WebKit's JSON.parse() didn't know how to handle when iterating and parsing through the settings keys in settings.js: migrateSettingsIfNecessary(settings).

2. WebKit's JSON.parse() struggled again in the same iterator when given an item that was not a stringified JSON object. So a simple validation was added in so only stringified JSON objects would be parsed.

After these fixes were made I tested it with Orion Version 0.99.126.4.1-beta (WebKit 618.1.2).

A small formatting fix was also made.

[tyirvine]

Hang on, sorry I just found an additional bug when using the / search command of Vimium. I'll work on fixing this.

TypeError: node.checkVisibility is not a function. (In 'node.checkVisibility()', 'node.checkVisibility' is undefined)