Fix underscore slash command autocomplete

[huntharo]

Summary

• allow _ while parsing slash-command autocomplete queries
• add regression tests for command autocomplete parsing in FoundationUtils
• keep autocomplete open for commands like /foo_bar

Testing

• swift test in packages/FoundationUtils
• manual verification in the macOS app: typing _ in a slash command no longer closes autocomplete

Screenshots

Before	After

Fixes #1350

[CLAassistant]

All committers have signed the CLA.

[huntharo]

Oh, it looks like this got released for me:

[mbproduct]

Thank you, i implemented it without accepting pull request. I think its already in release versions.