use same behavior as Gutenberg web for replacing an empty default block

[mzorz]

Comes from #327 (comment)

While PR #327 addresses #315 description, it's been observed that desired behavior is more that of following along Gutenberg web as much as we can. @koke observed the behavior should be closer to Gutenberg's by using isUnmodifiedDefaultBlock (see https://github.com/WordPress/gutenberg/blob/40d1282a992702e1933118ef626f0666692baf2b/packages/editor/src/components/inserter/menu.js#L389)

Please note that this makes the default block (currently, core/paragraph) the only block type eligible for replacement when such a block is both empty and selected, and the user taps on + to bring up the inserter's block picker.

With this PR then, if the user is standing on an empty Heading or Code block, they won't be able to replace such blocks anymore - the user will only be able to replace a block if they're standing on a Paragraph empty block.

I tested this on Gutenberg web and could observe the same.

To test: (paragraph)

1. load the demo app
2. tap on the + inserter icon to bring the BlockType picker up
3. choose Paragraph type (first on the left)
4. observe a new, empty Paragraph block is appended to the list and the editor scrolls there (the Paragraph editor gets the focus and the block is selected)
5. leaving the focus/selection there, tap on the + inserter icon again
6. now select T title block
7. observe the block is now a Title block type, as opposed to appending a new empty block once again.

To test: (other editables, non-paragraph)

8. leaving the focus/selection on the T block inserted in step 6 above:
9. tap on the + inserter icon
10. tap on any block
11. observe a new block is appended, instead of being replaced.

Try steps 8-10 again, inserting a new block while standing on a Code block instead of Title (Heading), and observe in both cases the new block is appended (or inserted below the selected block) rather than replace the previously selected block.

[koke]

I think you don't need the empty block check, since isUnmodifiedDefaultBlock already checks if the block has the default attributes

[mzorz]

addressed in 49e8630 - I still decided to keep isReplaceable() as a method because it makes it easier to read and follow the code where the method is called.

[Tug]

LGTM 👍

Unrelated to this PR: I noticed the Heading block fomatting doesn't work when starting from an empty block. I'll open an issue