Update code & streamline build

[Pathoschild]

This pull request...

• Updates the code for Stardew Valley 1.2 + SMAPI 1.14.
• Migrates to the mod build package.
  This automates references (so we no longer need to change reference paths in the .csproj), lets us compile the mod on Linux/Mac/Windows without changes, simplifies debugging from Visual Studio, and lets us deploy the mod into the game's Mods folder automatically.
• Adds manifest.json and icons to the project to simplify builds.
• Bumps the version for release.

If these changes look fine, can you release SkillPrestige 1.0.10.zip for players to use?

[Alphablackwolf]

Thank you, I had to merge this with my local manually and had some branching issues, so I'm closing the pull request, but the code was implemented.