Perform update in content rather than returning AST

[benmccann]

Something along the lines of:

{
  name: () => 'foo.svelte',
  content: ({ source })) => {
    const { script, css, template, generateCode } = parseSvelte(source);
        
    // do manipulations
    const ms = new MagicString(script.code);
    ms.overwrite(...);
    
    // or ast manipulation
    doStuff(script.ast);
    
    // performs a text replacement of `script`
    return generateCode({ script: ms.toString() });
    // or
    return generateCode({ script: script.generateCode() }); 
  }
},

This would allow us to touch only updated lines of code. There are currently 37 files written so is still pretty manageable.

This one would be important to figure out before release because we should make any breaking API changes before we start having community editors

We'd have to figure out the exact API. E.g. maybe it shouldn't be called generateCode but rather something like writeFile so that it can operate on files that might not be considered code. And the arg could be content rather than script for that reason