normalize svelte.config.js format

[Rich-Harris]

Changes

Currently, @snowpack/plugin-svelte will read configuration from a svelte.config.js file assuming the following structure:

module.exports = {
  preprocess: {...},
  ...compilerOptions
};

Unfortunately, this makes it impossible to add any config other than compiler options. For example, in the Snowpack/Svelte integration we're working on, it's necessary to specify an adapter to process the build output, and if we can't do so in svelte.config.js then we would have to introduce a new config file.

This PR changes the plugin behaviour to be in line with the format articulated here, which we're encouraging all members of the ecosystem to adopt:

module.exports = {
  preprocess: {...},
  compilerOptions: {...}
};

Unfortunately, I don't think there's a way to do this as a non-breaking change. (It should affect very few people, since most people only use svelte.config.js to specify preprocess, the behaviour of which is unchanged.)

Testing

Tested with a local project. There aren't any automated tests that currently cover this aspect of the plugin's behaviour.

Docs

The current expected svelte.config.js structure isn't documented anywhere, so this PR doesn't change anything.

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[FredKSchott]

Makes sense, and I'd rather make this change sooner than later.

Unfortunately this is currently documented in the plugin README and I know of a couple of projects that use it (including some of our own!). I'll go ahead and merge this and then spin up a follow-up PR to create warnings if you use the legacy format to help people transition.