Improved validation of empty or malformed `svelte.config.js`

[benmccann]

Is your feature request related to a problem? Please describe.

SvelteKit had an empty svelte.config.js in one of its test projects. It gave the error message:

│ Error while loading config
│ TypeError: Cannot read property 'compilerOptions' of undefined

Describe the solution you'd like

It'd be nice if it checked if the config existed before trying to read compilerOptions off it:

language-tools/packages/language-server/src/lib/documents/configLoader.ts

Line 159 in 7abeef4

...config.compilerOptions,

It'd also be nice if it printed the path to the svelte.config.js in the error. SvelteKit had tons of them in various test projects, so it took a little while to track down

Additional context
See sveltejs/kit#1988

[dummdidumm]

The error message should be better now.