Re-structure turbo-stream[action=morph] support

[seanpdoyle]

Related to hotwired/turbo#1240

Since the <turbo-stream action="morph"> hasn't yet been part of a release, there's an opportunity to rename it without being considerate of backwards compatibility.

As an alternative to introduce a new Stream Action, this commit changes existing actions to be more flexible.

For example, the <turbo-stream method="morph"> element behaves like a specialized version of a <turbo-stream method="replace">, since it operates on the target element's outerHTML property.

Similarly, the <turbo-stream method="morph" children-only> element behaves like a specialized version of a <turbo-stream method="update">, since it operates on the target element's innerHTML property.

-<turbo-stream action="morph">
+<turbo-stream action="replace" method="morph">
   <template>Replace me with morphing</template>
 </turbo-stream>

-<turbo-stream action="morph" children-only>
+<turbo-stream action="update" method="morph">
   <template>Update me with morphing</template>
 </turbo-stream>

[seanpdoyle]

I'd recently spent some time sharing news of the recent additions to Turbo, and had trouble differentiating morphing from the existing actions.

Explaining morphing in terms of Page Refresh replace versus morph and the [children-only] attribute in terms of updating an element gave me the idea to open this PR to consolidate the concepts.

@omarluq had you considered implementing morphing in this style as part of the initial implementation merged in hotwired/turbo#1185?

[omarluq]

@seanpdoyle Yes, I did consider this approach. The main reason I opted for a new action instead is due to the existence of the turbo-morph library, which many users are already familiar with. By matching the API, the new action would serve as a drop-in replacement.

However, I'm not opposed to the idea of consolidating the morph action as you suggested. Interestingly, I was asked the exact same question recently while explaining the feature at work. It seems there's a genuine interest in making these concepts more intuitive and unified, which I fully support.