Create a Pygments definition for FSH syntax highlighting

Transferred from internal JIRA issue originally created 2021-09-14.

This would allow FSH to be highlighted in tools like Zulip and perhaps would set the stage for including FSH source in IGs.
See: https://pygments.org/
and:
https://chat.fhir.org/#narrow/stream/179238-Zulip/topic/Custom.20code.20formatting

Start by reading the Pygments documentation and the linked Zulip discussion to understand how an FSH lexer should be registered and consumed. Done means FSH source is recognized and syntax-highlighted by tools such as Zulip, with the possible use in IGs considered.

Written by the indexing model from the issue text.