Support error codes

[Neil Mitchell]

Broken out from #7. @Mythra maintains a list of detailed descriptions for error codes for a project they are working on. These include error codes for Starlark errors. It would be great if Starlark could provide and integrate them, at low cost. The rough plan is:

• Each enum variant in our error enums becomes a named error code, e.g. ValueError::IntegerOverflow might get the code STARLARK::INTEGER_OVERFLOW. For Lint errors we convert the enum into a code in a similar manner, so that code/approach can be reused.
• We show those codes in the Diagnostic type, either optionally or always (we'll see how messy they make things, but probably always).
• @Mythra provides a markdown file with more information on each code. One potential format would be a single error_codes.md file with section headings ## STARLARK::INTEGER_OVERFLOW followed by arbitrary markdown content about the error.
• The Starlark library uses include_str!() to make that available, as a .describe(&self) method on Diagnostic, and also as a Diagnostic::describe_code(&str) method to enable implementing --describe=INTEGER_OVERFLOW on the command line.

While the steps are in logical order, in truth writing the Markdown file is the biggest effort by a huge margin, so that will probably come first - the actual technical tweaks are probably an hour in total, so I'll happily do them once a somewhat complete Markdown file arrives.