[lexical-markdown] Feature Change: Dont trim whitespaces on convertFromMarkdownString

[potatowagon]

Description

Issue #6325 is about inconsistent conversion regarding whitespace trim.

markdown -> string: does not trim whitespace
string -> markdown: trims whitespace

the whitespace trim was introduced in #2519 , which has a bunch of other markdown improvements.

One goal listed in PR2519 is to make markdown a bit closer to commonmark spec. checking commonmark, it doesnt trim whitespaces on string -> markdown conversion. same behavior for markdown on github. so i guess, it would be more commonmark-ish to not trim whitspaces.

hi @fantactuka ! May i check with you if theres any dangers to not trim whitespaces?

Closes #6325

Test plan

Before

Screen.Recording.2024-07-01.at.7.59.45.PM.mov

string -> markdown: trims whitespace

After

Screen.Recording.2024-07-01.at.7.59.34.PM.mov

string -> markdown: does not trim whitespace

---

npm run start & npm run test-e2e-chromium

no failing tests

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[github-actions]

size-limit report 📦

Path	Size
lexical - cjs	28.47 KB (0%)
lexical - esm	28.28 KB (0%)
@lexical/rich-text - cjs	36.86 KB (0%)
@lexical/rich-text - esm	28.08 KB (0%)
@lexical/plain-text - cjs	35.49 KB (0%)
@lexical/plain-text - esm	25.3 KB (0%)
@lexical/react - cjs	38.82 KB (0%)
@lexical/react - esm	29.27 KB (0%)

[fantactuka]

any dangers to not trim whitespaces

Can't recall specific reasons, so as long as tests are passing and we're closer to spec it should be good to go. I'd double check WP Composer once sync'd with www

[potatowagon]

tests failures unrelated, due to flaky