Allow specifying an entry point in programmatic usage

[metonym]

Ref: carbon-design-system/carbon-components-svelte#1636 (comment)

Currently, the getSvelteEntry relies purely on package.json for the Svelte entry point. It will throw an error if the "svelte" field is absent in the root package.json.

The plugin that allows programmatic usage should allow the entry point to be specified.

E.g.,

sveld({
  entry: "src/index.js",
  glob: true,
  markdown: true,
  markdownOptions: {
    onAppend: (type, document, components) => {
      if (type === "h1")
        document.append(
          "quote",
          `${components.size} components exported from ${pkg.name}@${pkg.version}.`
        );
    },
  },
  json: true,
  jsonOptions: {
    outFile: "docs/src/COMPONENT_API.json",
  },
});