add buildProject JS API, clean up config API

[FredKSchott]

/cc @drwpow, @Rich-Harris

Changes

• New buildProject() JS API.
  • this builds the entire project. If you need per-file builds, continue to use the dev server (startDevServer).
  • this allows for better programatic usage + helps us improve testing via the JS API. (/cc @drwpow)
• Cleans up config API (no breaking changes now, but loadAndValidateConfig() will be removed in v3.0 since it confuses usage with the much simpler createConfiguration() method).

Testing

• Covered by existing tests

Docs

• TODO

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[FredKSchott]

@drwpow once we start resolving by the project root (config file directory, instead of cwd) our tests can look something like this:

const config = snowpack.createConfiguration({root: TEST_DIR});
const {result} = await snowpack.buildProject({config});
expect(result['/path/to/output.js'].contents).toEqual(TEST_RESULT);

And then, we can look into creating a fixtures system for our most common tests (keeping the current full directory support for any too-complicated tests that need a full directory to express). Some examples:

// Example 1: Simplest form
// /test/build/fixtures/relative-import.json
{
  input: "import imgUrl from './test.png';",
  output: "import imgUrl from './test.png.proxy.js';", 
}

// Example 2: multi-file
// /test/build/fixtures/lazy-import.json
{
  input: {
    "file1.ts": "import './file2';",
    "file2.ts": "export const test: boolean = true;"
  }
  output: {
    "file1.js": "import './file2.js';",
    "file2.js": "export const test = true;"
  }
}

// Example 3: with simple config
// /test/build/fixtures/mount-simple.json
{
  config: {mount: {"src": "_dist_"}},
  input: {
    "src/file1.ts": "export const test: boolean = true;"
  }
  output: {
    "_dist_/file1.js": "export const test = true;"
  }

[Rich-Harris]

This is great news, since I'm currently trying to pass a --mount..foo/bar=/baz option (as in, .foo/bar should be mounted as /baz) but I can't work out how to get yargs parser to treat that as

mount: { '.foo/bar': '/baz' }

as opposed to

mount: { '': { 'foo/bar': '/baz' } }

whereas it will be trivial with the JS API.