fix empty line issue

[Shopiley]

Description

Currently, empty lines on markdown disappear when the markdown button is toggled.
This pull request aim to ensure empty lines are maintained upon toggling markdown.
I realised that setting a variable called shouldPreserveNewLines to true across all instances it was used seemed to fix the issue.

Closes 4838

Test plan

Before

Insert relevant screenshots/recordings/automated-tests

After

Insert relevant screenshots/recordings/automated-tests

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
lexical	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 29, 2024 5:25pm
lexical-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 29, 2024 5:25pm

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[github-actions]

size-limit report 📦

Path	Size
lexical - cjs	29.92 KB (0%)
lexical - esm	29.75 KB (0%)
@lexical/rich-text - cjs	38.57 KB (0%)
@lexical/rich-text - esm	31.65 KB (0%)
@lexical/plain-text - cjs	37.18 KB (0%)
@lexical/plain-text - esm	28.94 KB (0%)
@lexical/react - cjs	40.24 KB (0%)
@lexical/react - esm	33.01 KB (0%)

[etrepum]

The current default markdown behavior needs to be preserved for backwards compatibility reasons. The issue here is that it might be hard for users to discover this setting because there is a lack of documentation in the markdown package about this setting and the expected behavior. Unfortunately markdown is quite a complex specification mostly because there are many different ones (commonmark, github flavored markdown, mdx, other extensions etc.), many of them have conflicting requirements, and I think what has been accidentally created here is yet another "lexical markdown" spec which is not well defined but certain applications depend on exactly how it behaves now.

[Shopiley]

Okay, so what I have to do is experiment with this setting and document the expected behaviour.