ci: Add CI step to check if precompiled regl shaders need to be updated

[camdecoster]

Description

Add a CI step to check if precompiled regl shaders need to be updated.

Closes #7785.

Changes

• Update CI workflow
• Update instructions to reflect new process

Testing

• Create a draft PR that changes one of the files that would affect the shaders
• Review the workflow run

Notes

• This is currently a manual process and this (mostly) automates it

[emilykl]

Adding an additional line of explanation (feel free to adjust if not accurate)

Suggested change

	# stackgl_modules/ is listed here too because the regl-* shader libs live there;
	# changes can alter shader output and require regenerating the precompiled shaders.
	# If any of the directories listed below have changed, we need to re-run the regl-codegen step
	# stackgl_modules/ is listed here too because the regl-* shader libs live there;
	# changes can alter shader output and require regenerating the precompiled shaders.