Fix parsing of JSX with expression

[begleynk]

Disclaimer: I am not entirely sure that this is the correct fix, but it does pass the tests. Happy to make changes in the likely case I haven't quite understood this fully.

This fixes the issues mentioned in #136, where this case <x>{1}</x> was no longer parsed, and {1}<x/> caused a panic when converted into an AST (not an issue when converting to HTML).

I was first confused about what the expected output should be here, so I looked at micromark for inspiration. Here's what I found:

• Micromark parses <x>{1}</x> as a block level Flow element with an expression (unlike this crate as of alpha.16, which produced a paragraph with an inline JSX text node)
• Micromark parses {1}</x> into an AST with a root element, with two children: the expr, and the JSX tag

This change now makes this crate matches micromark's behavior.

Test program I used

import {mdxjs} from 'micromark-extension-mdxjs'
import {fromMarkdown} from 'mdast-util-from-markdown'
import {mdxFromMarkdown} from 'mdast-util-mdx'


const doc = "<span>{1}</span>";

const tree = fromMarkdown(doc, {
  extensions: [mdxjs()],
  mdastExtensions: [mdxFromMarkdown()]
})


console.log(JSON.stringify(tree, null, 2))

This also removes the MdxExpressionFlowNok part of the state, as it wasn't used any more.

Fixes #136

[wooorm]

thanks!