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/indexed_docs/src/providers/rustdoc.rs

@@ -53,7 +53,7 @@ impl IndexedDocsProvider for LocalRustdocProvider {
     }
 
     fn database_path(&self) -> PathBuf {
-        paths::support_dir().join("docs/rust/rustdoc-db.1.mdb")
+        paths::data_dir().join("docs/rust/rustdoc-db.1.mdb")
     }
 
     async fn suggest_packages(&self) -> Result<Vec<PackageName>> {
@@ -144,7 +144,7 @@ impl IndexedDocsProvider for DocsDotRsProvider {
     }
 
     fn database_path(&self) -> PathBuf {
-        paths::support_dir().join("docs/rust/docs-rs-db.1.mdb")
+        paths::data_dir().join("docs/rust/docs-rs-db.1.mdb")
     }
 
     async fn suggest_packages(&self) -> Result<Vec<PackageName>> {