fix: markdown import code block with prefix

[samuelint]

Problem

When triple backtick bloc code is prefix with tabs or spaces, the parser does not recognize text as a block code.

Not parsed:

   \```typescript
   const a = 'hello';
   \```

A concret use case, is when code block is under a list.
Example of markdown string to be loaded to lexical:

1. First step
   \```bash
   echo "hello"
   \```
2. Second step
   \```bash
   echo "world"
   \```

Note: \ is only there to escape code block rendering. The real use case does not have the \

Solution

Change CODE Transformer & MarkdownImport regex to accept tabs or spaces in prefix.

Current regex

Updated regex

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[acywatson]

I think this is OK. Thanks for the PR.

I will ask @fantactuka to take a look and merge if he has no concerns.

[samuelint]

Any update @acywatson @fantactuka, can this MR be merged?

[StyleT]

Merging this as apparently it got stuck :P