Review first: Remove .NET Standard 2.1 Dependency & Fix Scripting Restore

[RobProductions]

Fixes #111 as mentioned in #112 and also restores scripting backend after build as mentioned in #112.

Unfortunately since I use 2020.3 I can't even compile to test my other smaller feature branches without including the changes from this PR so the best I can do is to keep this as its own. The other PRs will have merged the commits from this PR but you can safely ignore that; once this is merged they won't appear as file differences I believe. Apologies for that!

[RobProductions]

Hey @robinnorth apologies for the persistence, but is there any update on getting these merged in? As it's been over a month now it's actually getting closer to beta build time for my game and I was hoping to use the new features here haha. That's besides the fact that I can't even use the latest version in my projects due to the errors which this PR fixes. I also had a few more ideas I wanted to tackle and submit as PRs but I'm waiting until we're sure that these can merge in successfully to avoid further complications/conflicts.

Thanks and let me know if there's anything else I can do to help with the process!

[robinnorth]

Hey @RobProductions, sorry for the delay and thanks for your patience. I've been incredibly busy this last month and haven't had a chance to review the changes yet. I will try and do so this week, however.