[M] Mount decimal.js in the sandbox and add a Decimal global

[evanrichards]

The host code works with decimal strings. Every embedder had to read the
decimal.js source from disk and mount it through the option nodeModules.
A file read does not work in a bundle or in a vm without a file system.

This change vendors the decimal.js source into the package as a string
constant. The virtual file system mounts it always, so the guest code runs
import Decimal from 'decimal.js' without an option. The new option
enableDecimalGlobal registers the class Decimal as a global, which
removes the need for an import statement in the guest code.

The package also exports the source as decimalJsSource. An embedder that
drives quickjs-emscripten-core directly gives the same class to the guest,
and needs no bundler rule to turn the file into text.

The script vendor.ts generates src/modules/generated/decimal.js from the
dev dependency decimal.js.

[evanrichards]

This change is part of the following stack:

• [M] Mount decimal.js in the sandbox and add a Decimal global #2 ◀

_{Change managed by git-spice.}