Support putting run configs in folders

[SquidDev]

• Remove unused RunConfig.genRuns.
• Rewrite templating to do a single find-and-replace, rather than a chain of them.
• Add a new ideConfigFolder option, which sets the folderName attribute on the IDEA config file.

I wasn't quite sure the best way to expose settings from RunConfigSettings, as it's currently a bit ad-hoc. For now, I've gone with the simplest option (a Property<String> getter), but happy to change that if preferred!

[modmuss50]

Looks like https://github.com/FabricMC/fabric-loom/blob/dev/1.10/src/test/groovy/net/fabricmc/loom/test/unit/IdeaClasspathModificationsTest.groovy is now failing

[SquidDev]

I did have a think about replacing IdeaClasspathModificationsTest.fromDummy with a call to RunConfig.fromDummy. Technically we can get away passing a null Project to it, but it felt a bit risky, so have left as-is for now.

[modmuss50]

I did have a think about replacing IdeaClasspathModificationsTest.fromDummy with a call to RunConfig.fromDummy. Technically we can get away passing a null Project to it, but it felt a bit risky, so have left as-is for now.

I'd leave it for now, the test coverage (and testability) of all of this could be improved vastly, but there is no point doing it as part of this PR. Ill take a closer look at the PR shortly and get it merged into the 1.11 branch.