Add detection of self hosted GitHub enterprise instances (#26482)

This PR does not close an issue, but it is an issue and and fix in one.
I hope this is ok, but please let me know if you prefer me to open an
issue before.

Release Notes:

- Add "copy permalink" action for self-hosted GitHub enterprise
instances

# Issue
### Related issues:
* https://github.com/zed-industries/zed/issues/26393
* https://github.com/zed-industries/zed/issues/11043

When you try to copy a permalink from a self-hosted GitHub enterprise
instance, you get the following error:

<img width="383" alt="permalink"
src="https://github.com/user-attachments/assets/b32338a7-a2d7-48fc-86bf-ade1d32ed1f7"
/>

You also cannot open a PR or commit when you hover over a git blame:


https://github.com/user-attachments/assets/a5491ce7-270b-412f-b9ac-027ec020b028


### Reproduce
If you do not have access to a self-hosted GitHub instance, you can
change the remote url of any git repo:
```
git remote set-url origin git@github.mycorp.com:nilskch/zed.git
```

With the fix, permalinks still won't bring you to a valid website, but
you can verify that they are correctly created.

# Solution

Currently, we only support detecting self-hosted GitLab instances, but
not self-hosted GitHub instances. We detect GitLab instances by checking
if "gitlab" is part of the git URL.

This PR adds the same logic to detect self-hosted GitHub enterprise
instances (by checking if "github" is in the URL).

This solution is not ideal, since self-hosted GitHub or GitLab instances
might not contain the word "github" or "gitlab". #26393 proposes adding
a setting that would allow users to map specific domains to their
corresponding git provider types. This mapping would help Zed correctly
identify the appropriate git instance, even if "gitlab" or "github" are
not part of the URL.

This PR does not implement the offered solution, but I added a TODO
where the fix for #26393 has to make changes.

crates/collab/src/tests/test_server.rs

@@ -271,7 +271,7 @@ impl TestServer {
 
         let git_hosting_provider_registry = cx.update(GitHostingProviderRegistry::default_global);
         git_hosting_provider_registry
-            .register_hosting_provider(Arc::new(git_hosting_providers::Github));
+            .register_hosting_provider(Arc::new(git_hosting_providers::Github::new()));
 
         let user_store = cx.new(|cx| UserStore::new(client.clone(), cx));
         let workspace_store = cx.new(|cx| WorkspaceStore::new(client.clone(), cx));