refactor(core): change _processedSpec to experimental_processedSpec

[etowahadams]

Fix #1068
Toward #

Change List

• Change _processedSpec in compile callback function to experimental_processedSpec

Checklist

• Ensure the PR works with all demos on the online editor
• Unit tests added or updated
• Examples added or updated
• Documentation updated (e.g., added API functions)
• Screenshots for visual changes (e.g., new encoding support or UI change on Editor)

[etowahadams]

In our discussion last week it seems like we settled on a new naming convention:

• For internally used names: Use underscores (for example_header),
• For externally used names: prefix with experimental, or wrap with experimental

Following this convention, I renamed _specProcessed to experimental_specProcessed. This is meant to be used externally, but it is unstable

[sehilyi]

cc'ing @thomcsmits for heads up