Ensure that `prettier` is run on CI (and actually run it)

[Shaptic]

Ideally we would be able to fully run yarn lint, but that would require a lot more code changes to accomplish, so this is a halfway measure. Closes #967.

[github-actions[bot]]

Size Change: -38 B (0%)

Total Size: 12.1 MB

Filename	Size	Change
dist/stellar-sdk.js	6.78 MB	-29 B (0%)
dist/stellar-sdk.min.js	5.34 MB	-9 B (0%)

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