Auto merge of #7165 - g-k:refactor-debug-options, r=ms2ger

Fail on unrecognized debug option

Refs: https://github.com/servo/servo/issues/7142

Ran some basic functional tests:

```
 $ ./mach run -d -Z bubble-widths,disable-canvas-aa,trace-layout tests/ref/blur_ref.html
 $ ./mach run -d -Z help
Usage: /Users/greg/servo/target/debug/servo debug option,[options,...]
	where options include

Options:
	bubble-widths                       Bubble intrinsic widths separately like other engines.
	disable-text-aa                     Disable antialiasing of rendered text.
	disable-canvas-aa                   Disable antialiasing on the HTML canvas element.
	dump-flow-tree                      Print the flow tree after each layout.
	dump-display-list                   Print the display list after each layout.
	dump-display-list-json              Print the display list in JSON form.
	dump-display-list-optimized         Print optimized display list (at paint time).
	relayout-event                      Print notifications when there is a relayout.
	profile-tasks                       Instrument each task, writing the output to a file.
	show-compositor-borders             Paint borders along layer and tile boundaries.
	show-fragment-borders               Paint borders along fragment boundaries.
	show-parallel-paint                 Overlay tiles with colors showing which thread painted them.
	show-parallel-layout                Mark which thread laid each flow out with colors.
	paint-flashing                      Overlay repainted areas with a random color.
	trace-layout                        Write layout trace to an external file for debugging.
	validate-display-list-geometry      Display an error when display list geometry escapes overflow region.
	disable-share-style-cache           Disable the style sharing cache.
	parallel-display-list-building      Build display lists in parallel.
	replace-surrogates                  Replace unpaires surrogates in DOM strings with U+FFFD. See https://github.com/servo/servo/issues/6564
	gc-profile                          Log GC passes and their durations.

$ ./mach run -d -Z blah
error: unrecognized debug option: blah
Servo exited with return value 1
```

Didn't check that setting debug flags actually did anything.
Haven't written much Rust so this feels more verbose than necessary.
Added `disable-canvas-aa` to debug options help.

Should DebugOptions struct derive Clone like Opts does?

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