Add some initial thoughts on the implementation.

src/domain/session/room/timeline/FORMATTED.md

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+# Ideas for formatted messages
+
+* Seems like a good idea to take some
+  inspiration from [Pandoc's AST](https://hackage.haskell.org/package/pandoc-types-1.22/docs/Text-Pandoc-Definition.html#t:Block).
+  Then, much like Pandoc AST, we can turn our representation into
+  markdown in the editor, or HTML in the view.
+* As such, we represent formatting by nesting parts
+  (we'd have a `ItalicsPart`, `BoldPart`, etc.)
+* We keep the "inline"/"block" distinction, but only
+  track it as a property, so that we can avoid adding
+  block parts to elements that cannot contain blocks
+  (headers, for instance, cannot contain blocks, but
+  lists -- themselves blocks -- can).
+* When parsing, we may need some sort of "permanent" context:
+  if we're parsing a header, and we are inside 3 layers of other
+  "inline" things (italics, strikethrough, and bold, for example),
+  and we encounter a block, that's still not valid.
+  * Element seems to not care at all about the validity of what
+    it's parsing. Do we assume the HTML is well-formatted, then?