feat: add beancount grammar

[KevinBatdorf]

This adds support for the beancount syntax

• I have read docs for adding a language.
• I have searched around and this is the most up-to-date, actively maintained version of the language grammar.
• I have added a sample file that includes a variety of language syntaxes and succinctly captures the idiosyncrasy of a language. See docs for requirement.

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[octref]

LGTM. You are sending a lot of good PRs. Thanks!

[KevinBatdorf]

You are sending a lot of good PRs. Thanks!

Yeah some easy ones :) When 1.0 is out I'll look to see how I can contribute something more substantial!