Custom VSCode theme

[kieranpotts]

References:

• https://code.visualstudio.com/api/references/theme-color
• https://code.visualstudio.com/docs/getstarted/themes

Consider dynamically generating theme, see https://github.com/primer/github-vscode-theme/tree/main for an example implementation.

Add Storybook for base color scales: https://primer.style/primitives/storybook/?path=/story/color-base-scales--all-scales&globals=theme%3Alight

[kieranpotts]

Related: https://github.com/kieranpotts/devtools/issues/9