To assist two+ active PRs (#2472 and #4037), this one does the following:

• Moves code related to creating & upgrading mod source folders out of UI and into a new separate Terraria.ModLoader.Core.SourceManagement class.
• Replaces textfile-generating hardcode with a filetree of embedded resources that are enumerated and exported to populate mod src folders. The embedded resources use the Mustache logicless template format in their names and contents (this concept will be nice to use for mod descriptions too in userland in the future). Optional files, such as the BasicSword, are skipped by making their filenames evaluate to nothing.
• Removes TargetFramework and PlatformTarget from default .csproj files, putting it into tModLoader.targets tMLMod.targets instead.
• Makes Csproj upgrades attempt to function via XML edits, if they're not too broken.
• When this hits Stable, update https://github.com/tModLoader/tModLoader/wiki/Basic-tModLoader-Modding-Guide#mod-skeleton-contents info and screenshots with new Content.Items namespace and folder.

Default mod template changes:

• launchSettings.json, uses tabs now.
• Csproj uses tabs now, no longer has an obsolete <?xml> header, is now practically empty besides for the targets import, with every other line just being nice "put this here" markups.
• The item template now uses the ExampleSword.png, which is nicer to look at. A tooltip has also been added, to immediately showcase how to make multiline strings in HJSON.
• The two .cs files now use file-scoped namespaces. The item template's code had its comments updated, and had magic numbers replaced with ID classes in a few lines.
• description.txt:

-{{ModDisplayName}} is a pretty cool mod, it does...this. Modify this file with a description of your mod.
+Modify this file with a description of your mod.

• As too many people don't bother even providing a mod icon, I felt like updating the default mod icon to be slightly more presentable, while communicating its placeholder nature:
  ->

Questionable notes:

• I've had to exclude the new template folder from AssemblyResourcesContentSource, because of it rather annoyingly crashing the game whenever it encountered a .cs + .png pair.
• The templates' EmbeddedResource LogicalNames are set to use forward slashes, due to localization files utilizing dots, which slashes get reasonlessly converted to by default. Ideally even the vanilla game would convert to using forward slashes, it's beneficial for its asset system.