Illegible captures in dark themes

[jiribenes]

Description

When using a default dark theme in VSCode, the captures are barely legible:

Dark+ / Dark Modern

Dark VS

Solutions

1. Change colour of captures (short-term, good first issue)

We should instead either allow the users to change the colour of captures, or use the vscode.window.activeColorTheme: ColorTheme API endpoint which can distinguish ColorTheme.kind \in { Dark, HighContrast, Light }.

We might also want to use the onDidChangeActiveColorTheme event to update this.

2. Use code inlays as a LSP feature, get captures "for free" (long-term, not a good first issue)

We could also instead use proper code inlays on the compiler / LSP server side, see effekt-lang/effekt#524.