Update for Stardew Valley 1.2, simplify code, and streamline build

[Pathoschild]

This pull request...

• Updates the code for Stardew Valley 1.2 + SMAPI 1.12.
• 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, and simplifies debugging from Visual Studio.
• Adds a build task to package the mod.
  This automatically copies the mod into the game's Mods folder, and creates the release structure in the solution bin folder for easier updates.
• Adds a .gitignore and removes build output (which shouldn't be in source control).
• Adds a .gitattributes to normalise line endings between Linux/Mac/Windows.
• Simplifies the code using a newer SMAPI event.
• Bumps the version for release.

If these changes look fine, can you release BirthdayMail 1.3.zip for players to use?

[Pathoschild]

This pull request is no longer merge-able, since you have changes on the main branches,