feat: add option to configure default image quality

[DSchau]

Fixes #4873

Don't love exporting the __DEFAULT_QUALITY__, but hey it works and makes it testable, so that's a win.

[pieh]

Nice! People actually really want that, so only nitpicky comment here is to add defaultQuality to resize transform

gatsby/packages/gatsby-transformer-sharp/src/extend-node-type.js

Line 392 in 1b2a4b6

defaultValue: 50,

and this is good to go

[KyleAMathews]

Hmm this still isn't global unfortunately as it only affects transformer-sharp and not remark-images or other plugins using plugin-sharp.

[pieh]

Hmm this still isn't global unfortunately as it only affects transformer-sharp and not remark-images or other plugins using plugin-sharp.

Given that gatsby-plugin-sharp isn't really gatsby plugin, but an utility library, to add this there, we would need to store default quality in some global state (either plugin or gatsby managed - cache util?). And also remove any graphql defaultValue as it will always override things

[DSchau]

Don't know how I missed these comments (maybe an e-mail filter or something) but I'll take a look at improving this soon.