Add support for configuring which versions of dependencies are used

[NullVoxPopuli]

Since we download from npm and support esm.sh, we'll also need to configure which strategy to use.

Config file could maybe be a second tab in the editor, and look something like this:

dependencies:
  svelte: "^5.0.0"
  react: "esm.sh/react"
  unified: "<bundled>"
  ember-source: "alpha"

Would use yaml with yaml_ls so folks don't have a bad time with the indentation-based config.

Here "<bundled>" would be the default / whatever is configured in the REPL app.

Using a top-level dependencies key like this also allows folks to customize / overrride the resolution behavior:

resolve:
  "@virtual/dep": "some-package"
  "@virtual/foo": "some-package/some/path"

though customizing resolve will be considered out of scope for now, until the need comes up.