fix: Improve flyout behavior

[gonfunko]

The basics

• I validated my changes

The details

Resolves

• Fixes Remove explicit variable handling from flyout #8547
• Fixes Dragging in from the flyout sometimes bumps unexpectedly #8769
• Fixes It's harder to drag out blocks if the flyout has a scrollbar #9239

Proposed Changes

This PR is a bit of a grab bag of flyout related fixes. Specifically, it:

• Removes a bunch of special handling of variable and block creation events when dragging a block out of the flyout. Experimentally this still seems to work fine, including undo, and including blocks that reference potential variables, and all tests still pass.
• Avoids a race condition that could result in inadvertent block bumps when dragging out a new block by calling appendInternal() instead of append(), the latter of which triggers an immediate (and unnecessary) render before the state of things is fully combobulated.
• Changes the behavior such that dragging on a block will not ever drag-to-scroll the flyout. The flyout can still be scrolled by clicking and dragging on the flyout workspace background, but clicks targeting blocks will always be treated as a drag on the block.