fix(gatsby-transformer-remark): fix spaces between text-bearing block-level elements in plain text excerpts

[mxxk]

Description

Fixes excerpt generation (in plain text mode) to add spaces between text-bearing block-level elements (i.e. paragraphs, headings, table cells) as well as line breaks.

That way, for the Markdown document

Paragraph 

# Heading

Broken  
Line

| TableCell |
| --------- |
| TableCell |

the plain-text excerpt becomes Paragraph Heading Broken Line TableCell TableCell instead of ParagraphHeadingBrokenLineTableCellTableCell.

Related Issues

Fixes #14528.

[mxxk]

cc @freiksenet

[Yihwan]

Woohoo!

[Yihwan]

@mxxk @freiksenet any chance this might get merged soon?

[mxxk]

@Yihwan I'm waiting on approval 🙂

[pieh]

Added tests are awesome! Let's ship it!

[gatsbot]

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[pieh]

Published [email protected]