Add support for Hack language

[mike-r-mclaughlin]

Requested by: https://github.com/sourcegraph/accounts/issues/2301 (part of https://github.com/sourcegraph/accounts/issues/6716) via Slack (see also)

Feature request description

Add support for Hack language. Priority is for syntax highlighting and search-based code intelligence.

Additional context

• Slack has open sourced a Treesitter parser for Hack
• See also: https://github.com/slackhq/vscode-hack/blob/master/syntaxes/hack.json