Change gatsby-transformer-remark excerpt behavior

[riywo]

Description

This commit changes excerpt behavior of gatsby-transformer-remark:

• Plain text even with separator
• New format for MARKDOWN

Previously, PLAIN excerpt returns different format with/without excerpt_separator because it respected gray-matter's excerpt (which is raw markdown) when excerpt_separator is specified. Now, it applies the same logic used by without separator.

To keep the current behavior somewhere, we also introduces MARKDOWN format. So, excerpt(format: MARKDOWN) + excerpt_separator is identical with the current excerpt(format: PLAIN) + excerpt_separator.

Related Issues

Addresses #4459

[riywo]

I have no idea why some of tests are failing.

[freiksenet]

There was a stale snapshot (happens sometimes when writing tests). I've removed it now.

I'll merge it and release as a minor version update for the plugin, thanks a lot!

[gatsbot]

Holy buckets, @riywo — we just merged your PR to Gatsby! 💪💜

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Thanks again!

[freiksenet]

Published as 2.4.0. Thank as a lot! 💜 💪

[riywo]

OMG, thank you so much!!!

[zslabs]

cc @wardpeet #15286