Fix CI setup for formatting

[colecrouter]

@Leftium found out that the current format script is running both BiomeJS & Prettier on Svelte/Astro files (see this link from Biome docs). This is fine but prevents format:check from succeeding.

This commit adds an override to the BiomeJS config, which skips formatting for:

• Semi-supported file formats
• Files we use other formatters on

[colecrouter]

For clarity: