Update for SMAPI 2.0 and streamline build

[Pathoschild]

This pull request...

• Updates the code for compatibility with SMAPI 2.0 (without removing compatibility with SMAPI 1.x).
• 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.
• Loads the heart texture with SMAPI's new content API, which is much easier to use.
• Adds a .gitignore and removes files which don't need to be in source control.
• Adds a .gitattributes to normalise line endings between Linux/Mac/Windows.
• Refactors and documents the code a bit.