erlang: Add Erlang Language Platform support (#14879)

Added support for the [Erlang Language
Platform](https://whatsapp.github.io/erlang-language-platform/) language
server to the Erlang extension.

Release Notes:

- N/A

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

extensions/erlang/src/language_servers/erlang_language_platform.rs

@@ -0,0 +1,33 @@
+use zed_extension_api::{self as zed, LanguageServerId, Result};
+
+pub struct ErlangLanguagePlatform;
+
+impl ErlangLanguagePlatform {
+    pub const LANGUAGE_SERVER_ID: &'static str = "elp";
+
+    pub fn new() -> Self {
+        Self
+    }
+
+    pub fn language_server_command(
+        &mut self,
+        language_server_id: &LanguageServerId,
+        worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        Ok(zed::Command {
+            command: self.language_server_binary_path(language_server_id, worktree)?,
+            args: vec!["server".to_string()],
+            env: Default::default(),
+        })
+    }
+
+    fn language_server_binary_path(
+        &mut self,
+        _language_server_id: &LanguageServerId,
+        worktree: &zed::Worktree,
+    ) -> Result<String> {
+        worktree
+            .which("elp")
+            .ok_or_else(|| "elp must be installed and available on your $PATH".to_string())
+    }
+}