Detect default themes

[vocksel]

Problem

While it's easy enough to detect the themes the user has installed, there are several default themes that VS Code comes with that we do not detect.

Solution

Add support for finding the install directory for VS Code on the user's system to grab the default theme files.

Closes #3

This has some issues currently. "Default Dark+" currently throws an error right now. Others like Monokai work perfectly fine though!

[vocksel]

I have tested this and it can detect default extensions on both Windows and MacOS. These are the paths used to accomplish this:

export const WINDOWS_DEFAULT_EXTENSIONS = [
    process.platform === 'win32' && path.join(process.env.LOCALAPPDATA,
        '/Programs/Microsoft VS Code/resources/app/extensions'),
    'C:/Programs Files/Microsoft VS Code/resources/app/extensions',
    'C:/Program Files (x86)/Microsoft VS Code/resources/app/extensions'
]

export const MACOS_DEFAULT_EXTENSIONs = [
    '/Applications/Visual Studio Code.app/Contents/Resources/app/extensions'
]

I'm wondering how brittle these paths are though.