feat: export sveld for programmatic usage

[metonym]

Closes #65

Future work:

• allow the input to be passed as an option (currently, it will still use package.json#svelte

const { sveld } = require("sveld");
const pkg = require("./package.json");

sveld({
  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",
  },
});