fix(gatsby-remark-images): Escape HTML characters for image caption

[joshuaavalon]

Description

Escape HTML character(s) in alt attribute of <img>.

Related Issues

#14495

[freiksenet]

@joshuaavalon Thanks a lot! Could you run prettier on the code to fix style errors?

[superhawk610]

LGTM 👍

[gatsbot]

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

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[freiksenet]

Thank you @joshuaavalon <3!