Add support for hard tabs

* Add a `hard_tabs` setting that causes indentation to be performed
  using a tab instead of multiple spaces.
* Change Buffer's indentation-related APIs to return an `IndentSize`
  struct with a length and a kind, instead of just a single u32.
* Use hard tabs by default in Go.

crates/zed/src/main.rs

@@ -94,6 +94,14 @@ fn main() {
                 ..Default::default()
             },
         )
+        .with_overrides(
+            "Go",
+            settings::LanguageOverride {
+                tab_size: Some(4),
+                hard_tabs: Some(true),
+                ..Default::default()
+            },
+        )
         .with_overrides(
             "Markdown",
             settings::LanguageOverride {