Allow users to modify settings by json

[R-NK]

Summary of the Pull Request

Currently, users have to modify the source to add apps or change the default setting.
This PR allows users to do that by creating a json file like below:

config.json

{
    "Apps": [
        {
            "ProcessName": "WindowsTerminal",
            "ExeFilePath": "wt.exe",
            "ModifierKeys": 1,
            "Keys": 84
        },
        {
            "ProcessName": "Spotify",
            "ExeFilePath": "Spotify.exe",
            "ModifierKeys": 1,
            "Keys": 83
        }
    ]
}

The json should be located at:

1. same directory as WindowsTermialTray.exe
2. $env:LOCALAPPDATA\WindowsTermialTray\config.json

Checklist

• Implement Config class with builder pattern
• Add a project for unit tests (I'm not familiar with C#, so I followed Microsoft's docs for unit test)
• Change default key binding to T from oemtilde which only available on US Keyboard.

and just a trivial question, which is the correct application name, TermialTray or TerminalTray? Both of them are found at the source.