Improve block parsing for alphanumeric component names

[johno]

Component names and HTML can be alphanumeric. This
updates the block parsing regex to reflect that.

Closes #593

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[chasemccoy]

@johno For what it's worth, I was seeing the issue in #593 without alphanumeric tags.

My code is essentially like this:

<Paragraph bg='red.500' color='white'>Foo...</Paragraph>

And that was rendering as two p tags, one inside the other.

[wooorm]

Does this solve the issue? 🤔 h1 is supported in remark as HTML, so maybe it’s something else?

Could a failing/fixed test help?

Note there’s two elements:

mdx/packages/remark-mdx/block.js

Lines 50 to 51 in 4599f97

	[elementOpenExpression, elementCloseExpression, true],
	[otherElementOpenExpression, elementCloseExpression, false]

[johno]

Does this solve the issue? 🤔 h1 is supported in remark as HTML, so maybe it’s something else?

MDX is overriding the remark HTML handling so that we can handle JSX syntax which is more flexible. It now should solve the issue with my latest changes this morning.

Could a failing/fixed test help?

Added a bunch more test cases 💟

[ChristianMurphy]

LGTM, thanks @johno! 🙇