Feat: Peritext-like rich text support

[zxch3n]

This PR implements a new rich text CRDT algorithm that is greatly inspired by Peritext.

It behaves just like Peritext when editing locally. The behavior only differ when there is concurrent insertion at the style boundaries.

It satisfies the requirements listed in Peritext paper Chapter 3 "CRITERIA FOR INTENT PRESERVATION". Specifically, it passes all the following tests besides the consistency fuzzing tests.

Richtext Test Cases

0. Insert into bold span

Name	Text
Origin	Hello World
Concurrent A	<b>Hello World</b>
Concurrent B	Hello New World
Expected Result	<b>Hello New World</b>

1. Merge Concurrent Styles

Name	Text
Origin	Hello World
Concurrent A	<b>Hello</b> World
Concurrent B	Hel<b>lo World</b>
Expected Result	<b>Hello World</b>

2. Concurrent insert text & remove style

Name	Text
Origin	<b>Hello World</b>
Concurrent A	Hello <b>World</b>
Concurrent B	<b>Hello a World</b>
Expected Result	Hello a <b>World</b>

3. Concurrent insert text & style

Name	Text
Origin	Hello World
Concurrent A	Hello <b>World</b>
Concurrent B	Hello a World
Expected Result	Hello a <b>World</b>

4. Concurrently insert text and style that expands

This doesn't work well in our implementation

Name	Text
Origin	Hello World
Concurrent A	<b>Hello</b> World
Concurrent B	Hello* World
Expected Result	<b>Hello*</b> World

5. Concurrent text edit & style that shrink

Name	Text
Origin	Hello World
Concurrent A	<link>Hello</link> World
Concurrent B	Hey World
Expected Result	<link>Hey</link> World

6. Local insertion expand rules

Hello World

When insert a new character after "Hello", the new char should be bold but not link

Hellot World

Name	Text
Origin	<b><link>Hello</link><b> World
Expected Result	<b><link>Hello</link>t<b> World

7. Concurrent unbold

In Peritext paper 2.3.2

Name	Text
Origin	<b>The fox jumped</b> over the dog.
Concurrent A	The fox jumped over the dog.
Concurrent B	<b>The </b>fox<b> jumped</b> over the dog.
Expected Result	The fox jumped over the dog.

8. Bold & Unbold

In Peritext paper 2.3.3

Name	Text
Origin	<b>The fox jumped</b> over the dog.
Concurrent A	<b>The fox</b> jumped over the dog.
Concurrent B	<b>The</b> fox jumped over the <b>dog</b>.
Expected Result	<b>The</b> fox jumped over the <b>dog</b>.

9. Overlapped formatting

In Peritext paper 3.2, example 3

Name	Text
Origin	The fox jumped.
Concurrent A	The fox jumped.
Concurrent B	The fox jumped.
Expected Result	The fox jumped.

10. Multiple instances of the same mark

[Leeeon233]

what's the meaning of INDEX_INTERVAL?