fix: add table variant for require.git field in lakefile.toml schema (#11536)

This PR corrects the JSON Schema at
`src/lake/schemas/lakefile-toml-schema.json` to allow the table variant
of the `require.git` field in `lakefile.toml` as specified in the
[reference](https://lean-lang.org/doc/reference/latest/Build-Tools-and-Distribution/Lake/#Lake___Dependency-git).

Closes #11535

src/lake/schemas/lakefile-toml-schema.json

@@ -355,7 +355,8 @@
                     "description": "A dependency on the local filesystem, specified by its path."
                 },
                 "git": {
-                    "type": "string",
+                    "type": ["string", "object"],
+                    "default": "",
                     "description": "A dependency in a Git repository, specified by its URL."
                 },
                 "subDir": {