Move template part replacement flow to the transform menu

[jameskoster]

The template part replacement flow is currently initiated from the ellipsis menu on the block toolbar.

Conceptually this is closely related to transforms, and so could be moved to that menu instead, something like:

cc @Mamaduka

[carolinan]

Should this implementation be moved to the blockswitcher component?

[Mamaduka]

@carolinan, that's correct. Primarily in BlockSwitcherDropdownMenu component.

[carolinan]

One of the reasons why I was hesitating to do that is that the BlockSwitcherDropdownMenu is in the block-editor package.
We will be checking if every loaded block is a template part, even though template parts are only available in the site editor? 🤔

[annezazu]

Noting that this came up as part of the FSE Outreach Program's Build a Block Theme exploration:

Switching a Header Template. What about using the Transform To menu to switch a Reusable block to another Reusable block, and one type of pattern with another similar type. Be it Header, Footer etc. Being able to easily transform from one type of Header pattern to another.

While I noted the current "replace" flow and the work being done here #44582, I wanted to connect the dots to this specific part of the feedback.

[paaljoachim]

Thanks Anne!

Btw
Here is another approach:
A cycle between patterns icon in the Toolbar. I am also suggesting a broader solution.
#44581 (comment)