Bump postcss and pxt-core

[dependabot[bot]]

Removes postcss. It's no longer used after updating ancestor dependency pxt-core. These dependencies need to be updated together.

Removes postcss

Updates pxt-core from 3.12.9 to 8.3.1

Commits

• 4169d90 8.3.1
• 4f6bd40 Bumping minor verstion to 8.3.0
• db9b0b2 8.2.17
• 78d9cdd Add multiplayer app, adjust package dependencies (#9051)
• 92e7048 8.2.16
• 0237301 Quick tutorial updates from testing (#9050)
• 8e30a88 8.2.15
• 4106dae Inline snippet click highlight (#9049)
• 8fb58ab Fix Image Editor Top Bar Button Sizing (#9054)
• 75404b1 Anzhou/localization blog (#9052)
• Additional commits viewable in compare view

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