[lexical-markdown] Feature: add ability to hook into the import process for multiline element transformers

[AlessioGr]

Description

This PR is similar to #6681 but for multiline element transformers.

This adds a new handleImportAfterStartMatch property to MultilineElementTransformer that allows you to replace whatever happens after the regExpStart has matched with your own, custom logic.

This is great for cases where the default logic for finding the end of a given match and running the replace function is not enough.

E.g. for JSX elements, we need custom logic to keep track of the amount of opened and closed tags matched in order to find the correct, ending tag. Simply looking for the next closing tag using regExpEnd would not work for nested JSX elements. (Example of this in action)

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Name	Status	Preview	Comments	Updated (UTC)
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[github-actions]

size-limit report 📦

Path	Size
lexical - cjs	29.94 KB (0%)
lexical - esm	29.81 KB (0%)
@lexical/rich-text - cjs	38.54 KB (0%)
@lexical/rich-text - esm	31.61 KB (0%)
@lexical/plain-text - cjs	37.15 KB (0%)
@lexical/plain-text - esm	28.99 KB (0%)
@lexical/react - cjs	40.34 KB (0%)
@lexical/react - esm	33.08 KB (0%)

[AlessioGr]

Hi @potatowagon, just added a test for this feature. Not the simplest one, but an interesting use-case:

By default, this would create 2 code blocks. In the newly added test, we use handleImportAfterStartMatch to add open / end regex counting in order to find the correct end match. The result is just one code block that looks like this