Synchronise with svelte.config.js

Two parts to this:

• the plugin should accept a compilerOptions property, instead of just throwing top-level options at the compiler (we can detect unexpected top-level properties and warn accordingly, so while this will be a breaking change — which is fine, since we’re about to release a major — it needn’t be a confusing one)
• it should load a svelte.config.js if one exists. (do we allow config to come from other places, like svelte.config.json? my gut says no, keep it simple)

[lukeed]

ESM files can still require CommonJS files. It just doesn’t work the other way around

[dummdidumm]

A question about importing the svelte.config.js: right now this config has to be written in commonjs style because it’s loaded by the language server through a dynamic require. AFAIK you can write rollup configs using esm style. Is that a problem?