[WIP] Align pretty-printed diagnostic messages with source code that uses horizontal whitespace characters other than space (U+0020)

[WowbaggersLiquidLunch]

I noticed a few problems with the pretty-printed diagnostics.

Given a short Tokamak program as follows:

import TokamakShim

struct TokamakApp: App {
	var body: some Scene {
		WindowGroup("Tokamak App") {
			ContentView()
		}
	}
}

struct ContentView: View {
	var body: some View {
		VStack {
			Button("foo", action: nil)
			Button("字符串", action: nil)
		}
	}
}

Ni!

TokamakApp.main()

carton dev provides the following diagnosis:

The most noticeable problem in the diagnostics is that the "^"s don't point to where the errors actually are. This pull request intends to address this problem by first stripping all leading whitespace in the pretty-printed source code, then copying any remaining whitespace verbatim to the indentation before each "^".

Some other problems include duplicated error messages and out-of-order diagnosis. I intend to address them in follow-up PRs.

[WowbaggersLiquidLunch]

There are still some test cases to be added.

[WowbaggersLiquidLunch]

There isn't much documentation to go by in the source. @carson-katri, since you are the author of #122, it will be great if you could take a look at the changes and see if things are done correctly.

[carson-katri]

LGTM except for the few linter errors, and the placeholders in the flush function's doc comment. Thanks for the fix!