Remove ENABLE_MATH option from pulldown_cmark to fix links which contain dollar sign

[spotikhanov]

This pr closes #21466 issue by disabling math in pulldown_cmark Parser.

The dollar sign symbol is used in pulldown_cmark Math extension, see "Math in links" section for more details: https://pulldown-cmark.github.io/pulldown-cmark/specs/math.html

I've tried another approach at first, without disabling math extension:

let iterator = TextMergeWithOffset::new(Parser::new_ext(text, options));

instead of current implementation

Parser::new_ext(text, options).into_offset_iter()

This way pulldown_cmark merges consecutive text events and this helps to correctly parse links from plain text: https://svelte.dev/docs/svelte/$state

But in this case the dollar sign still breaks markdown links: [https://svelte.dev/docs/svelte/$state](https://svelte.dev/docs/svelte/$state)

So in the end I disabled the math extension, it fixes both link formats. See markdown/examples/markdown.rs to reproduce.

Release Notes:

• N/A

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[spotikhanov]

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	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[cole-miller]

Makes sense to me since we don't handle the Math events from pulldown_cmark at all. Thanks!