editors/zed: Fix WASM extension host panic on FreeBSD ("unsupported os")

Zed's WASM extension host panics with "unsupported os" on FreeBSD when
any Rust-based extension (Typst, LaTeX, HTML, typos, etc.) loads. The
panic occurs at
crates/extension_host/src/wasm_host/wit/since_v0_8_0.rs:840 where
current_platform() only handles macOS, Linux, and Windows.

The patch maps FreeBSD to the Linux OS variant and seems to solve the
issue, allowing the user to install the extensions for Typst, LaTex,
and typos.

PR: 293816
Reported by: John Holloway <mail@jholloway.dev>
Pull Request: https://github.com/tagattie/FreeBSD-Zed/pull/3