This PR fixes #4043.

The minified D3 bundle (d3.min.js) previously included non-ASCII characters
such as µ and −. When these characters appear in a minified JavaScript
bundle, browser engines must represent the entire file as a 2-byte UTF-16
string rather than a 1-byte Latin-1 string, which increases memory usage and
slows down parsing.

This PR updates the Rollup Terser configuration so that the minified output is
ASCII-only.

What was changed
Terser defaults to ascii_only: false. This allows Unicode characters to pass
through during minification. To enforce ASCII-only output:

• format.ascii_only: true has been added to the Terser configuration.
• The previous use of the output key has been merged into the format block,
  because Terser requires either format or output (not both).
• The preamble banner is now emitted via format.preamble.

Updated terser() configuration:

terser({
  format: {
    preamble: config.output.banner,
    ascii_only: true
  },
  mangle: {
    reserved: ["InternMap", "InternSet"]
  }
})