fix(gatsby-transformer-remark): Handle headings with nested text

[diegohaz]

Description

Here's a fix. If there's a better way to write the solution, just let me know.

Related Issues

Fixes #11879

[DSchau]

Nice idea here with this failing test 👏

[stefanprobst]

I'm sorry I missed this one before #11888
Anyway, we should make sure headings like the following are handled correctly:

# An **important** heading with `inline code` and text

[stefanprobst]

@diegohaz I'll close #11888 in favor of this. We probably want to use mdast-util-to-string here, which handles formatted text as well as inlineCode.

[diegohaz]

Thank you @stefanprobst. Added a test case with your example and fixed with mdast-util-to-string.

[sidharthachatterjee]

Thank you @diegohaz 👍

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