Add --user-data-dir CLI flag and propose renaming support_dir to data_dir (#26886)

This PR introduces support for a `--user-data-dir` CLI flag to override
Zed's data directory and proposes renaming `support_dir` to `data_dir`
for better cross-platform clarity. It builds on the discussion in #25349
about custom data directories, aiming to provide a flexible
cross-platform solution.

### Changes

The PR is split into two commits:
1. **[feat(cli): add --user-data-dir to override data
directory](https://github.com/zed-industries/zed/pull/26886/commits/28e8889105847401e783d1739722d0998459fe5a)**
2. **[refactor(paths): rename support_dir to data_dir for cross-platform
clarity](https://github.com/zed-industries/zed/pull/26886/commits/affd2fc606b39af1b25432a688a9006229a8fc3a)**


### Context
Inspired by the need for custom data directories discussed in #25349,
this PR provides an immediate implementation in the first commit, while
the second commit suggests a naming improvement for broader appeal.
@mikayla-maki, I’d appreciate your feedback, especially on the rename
proposal, given your involvement in the original discussion!

### Testing
- `cargo build `
- `./target/debug/zed --user-data-dir ~/custom-data-dir`

Release Notes:
- Added --user-data-dir CLI flag

---------

Signed-off-by: Marko Kungla <marko.kungla@gmail.com>

crates/zed/src/main.rs

@@ -172,6 +172,11 @@ fn fail_to_open_window(e: anyhow::Error, _cx: &mut App) {
 fn main() {
     let args = Args::parse();
 
+    // Set custom data directory.
+    if let Some(dir) = &args.user_data_dir {
+        paths::set_custom_data_dir(dir);
+    }
+
     #[cfg(all(not(debug_assertions), target_os = "windows"))]
     unsafe {
         use windows::Win32::System::Console::{ATTACH_PARENT_PROCESS, AttachConsole};
@@ -962,6 +967,14 @@ struct Args {
     /// URLs can either be `file://` or `zed://` scheme, or relative to <https://zed.dev>.
     paths_or_urls: Vec<String>,
 
+    /// Sets a custom directory for all user data (e.g., database, extensions, logs).
+    /// This overrides the default platform-specific data directory location.
+    /// On macOS, the default is `~/Library/Application Support/Zed`.
+    /// On Linux/FreeBSD, the default is `$XDG_DATA_HOME/zed`.
+    /// On Windows, the default is `%LOCALAPPDATA%\Zed`.
+    #[arg(long, value_name = "DIR")]
+    user_data_dir: Option<String>,
+
     /// Instructs zed to run as a dev server on this machine. (not implemented)
     #[arg(long)]
     dev_server_token: Option<String>,