lsp-config: Allow setting a server's environment variables (#27213)

Closes https://github.com/zed-industries/zed/issues/14334, allowing
users to set environment variables for a language server binary like:

```json
"lsp": {
  "rust-analyzer": {
    "binary": {
      "path": "/Users/dbarsky/.cargo/bin/rust-analyzer",
      "env": {
        "RA_PROFILE": "*>100"
      }
    },
  }
}
```

The newly introduced environment variables are merged with the shell
environment. Perhaps more controversially, I've _also_ removed the
trimming/`stderr:`-prefixing of language server logs. This because
rust-analyzer has some nice, tree-shaped profiling built-in, and it
prevents us from printing profiles like this:

<details>
<img width="1147" alt="Screenshot 2025-03-20 at 12 09 14 PM"
src="https://github.com/user-attachments/assets/b7066651-6394-492b-b745-906c66d3c7b2"
/>
</details>

Release Notes:

- Added the ability to set a language server's environment variables.
- Removed the `stderr`-prefix of a language server's stderr logs.

crates/language_tools/src/lsp_log.rs

@@ -540,7 +540,6 @@ impl LogStore {
             IoKind::StdOut => true,
             IoKind::StdIn => false,
             IoKind::StdErr => {
-                let message = format!("stderr: {}", message.trim());
                 self.add_language_server_log(language_server_id, MessageType::LOG, &message, cx);
                 return Some(());
             }