Full bidirectional (RTL) support

The following is a short overview of bi-directional editing surfaces and my suggestion for improving Lexical’s implementation. _(Continued from the [Discord discussion](https://discord.com/channels/953974421008293909/955972012541628456/993985520747155476))_

### Overview
Modern editing surfaces’ default behavior today is to support bi-directional content by setting the direction based on the first letter’s language. This can be seen in the current Lexical implementation, _Apple Notes_, _Gmail_, and _Google Docs_:

https://user-images.githubusercontent.com/1909713/177728347-054678d1-cf3e-47d5-817e-c5041ff144ef.mp4

One implementation detail crucial for longer-form and complex content (like a letter or an article) is maintaining the previous paragraph setting if the user manually sets it. _Microsoft Office Word_ gets this correctly:

https://user-images.githubusercontent.com/1909713/177722676-d603b861-daad-42b4-8378-f053bc455fa9.mp4

_The recordings demonstrate Hebrew, but the behavior is similar in Arabic._

### Implementation Suggestions

The following are my suggestions for scalable bi-directional support in Lexical:
1. Maintain the current dynamic evaluation of direction as the default behavior.
2. Add support for manually toggling/setting the node’s direction.
3. If a node has a `direction` manually set, it should be respected. (When parsing a serialized editor state, the current implementation dynamically evaluates the content, disregarding the provided configuration.)
4. One (or both) of the following will allow composing more complex content (letter/article) without constant friction:
4.1. If the root node’s direction is _manually_ set, it should be the default direction for child nodes.
4.2. When creating a following paragraph, if the previous paragraph has a manual direction set, it should be copied into the next paragraph.

The issue does not name files or tests. Start by locating Lexical’s existing dynamic direction evaluation and serialized editor-state parsing, then trace how root and paragraph directions are created and preserved. Done means manually set directions are supported, survive parsing, and follow the specified root and next-paragraph behavior.

Written by the indexing model from the issue text.