Add excalidraw lint check

[russellb]

Diagrams generated with excalidraw and exported as a png have metadata embedded in the image file that allow them to be loaded back into the tool for further editing. The metadata is included by default, but there's an option to turn it off. To help ensure we don't accidentally lose the metadata in some iteration of a diagram, a lint check can be added that runs in a github action to ensure the data is present.

For reference, an equivalent check implemented as a Makefile target:

.PHONY: png-lint
png-lint: dist/.png-lint ## Lint the png files from excalidraw

dist/.png-lint: $(shell find . -iname '*.png') | dist
	$(ECHO_PREFIX) printf "  %-12s ./...\n" "[PNG LINT]"
	$(CMD_PREFIX) for file in $^; do \
		if echo "$$file" | grep -q --basic-regexp --file=.excalidraw-ignore; then continue ; fi ; \
		if ! grep -q "excalidraw+json" $$file; then \
			echo "$$file was not exported from excalidraw with 'Embed Scene' enabled." ; \
			echo "If this is not an excalidraw file, add it to .excalidraw-ignore" ; \
			exit 1 ; \
		fi \
	done
	$(CMD_PREFIX) touch $@

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