src: print string content better in BlobDeserializer

When it's a short string, print it inline, otherwise print it
from a separate line. Also add the missing line breaks finally.

sea: support embedding assets

With this patch:

Users can now include assets by adding a key-path dictionary
to the configuration as the assets field. At build time, Node.js
would read the assets from the specified paths and bundle them into
the preparation blob. In the generated executable, users can retrieve
the assets using the sea.getAsset() and sea.getAssetAsBlob() API.

{
  "main": "/path/to/bundled/script.js",
  "output": "/path/to/write/the/generated/blob.blob",
  "assets": {
    "a.jpg": "/path/to/a.jpg",
    "b.txt": "/path/to/b.txt"
  }
}

The single-executable application can access the assets as follows:

const { getAsset } = require('node:sea');
// Returns a copy of the data in an ArrayBuffer
const image = getAsset('a.jpg');
// Returns a string decoded from the asset as UTF8.
const text = getAsset('b.txt', 'utf8');
// Returns a Blob containing the asset without copying.
const blob = getAssetAsBlob('a.jpg');

Drive-by: update the documentation to include a section dedicated
to the injected main script and refer to it as "injected main
script" instead of "injected module" because it's a script, not
a module.

Refs: nodejs/single-executable#68