Allow the pattern block to pull from original source until it's modified

[jffng]

What problem does this address?

The pattern block works by replacing the block's contents with the contents of the pattern on initial load. The pattern will stop being used when the template or template part where it's used is modified.

See #36090 (comment) for more context.

What is your proposed solution?

Explore a way to preserve the pattern unless the pattern's contents are modified. This was how the block initially worked, but it required an undesirable wrapping element.

[MaggieCabrera]

Part of #39281 is relevant to this