Inline inject that supports sideEffects metadata

[ggoodman]

The current inject feature is very powerful but requires files to be available on disk to leverage.

I would like to suggest the option of supporting an alternative inject specifier format that allows the contents of the injected file to be specified inline. In order to also be able to benefit from the tree-shaking applicable to side-effect-free modules, it would also be helpful to be able to specify this metadata inline.

Illustration of the general idea:

await build({
  // ...
  inject: [
    {
      content: 'export const foo = "bar";',
      // optional, for debug and error info
      fileName: 'inject-bar-foos.js',
      // optional, for determining if it can be safely pruned when no exported symbols are referenced
      sideEffects: false,
    },
  ],
  // ...
});