subscriber: add ability to disable ANSI without crate feature

Currently it is not possible to disable ANSI in `fmt::Subscriber`
without enabling the `ansi` crate feature. This makes it difficult for
users to implement interoperable settings that are controllable with
crate features without having to pull in the dependencies `ansi` does.

This adds a simple `fmt::Subscriber::disable_ansi()` that is available
even without the `ansi` crate feature.

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -284,6 +284,14 @@ impl<C, N, E, W> Subscriber<C, N, E, W> {
         }
     }
 
+    /// Disable ANSI terminal colors. Available even without the `ansi` crate feature.
+    pub fn disable_ansi(self) -> Self {
+        Subscriber {
+            is_ansi: false,
+            ..self
+        }
+    }
+
     /// Sets whether to write errors from [`FormatEvent`] to the writer.
     /// Defaults to true.
     ///

tracing-subscriber/src/fmt/mod.rs

@@ -620,6 +620,14 @@ where
         }
     }
 
+    /// Disable ANSI terminal colors. Available even without the `ansi` crate feature.
+    pub fn disable_ansi(self) -> CollectorBuilder<N, format::Format<L, T>, F, W> {
+        CollectorBuilder {
+            inner: self.inner.disable_ansi(),
+            ..self
+        }
+    }
+
     /// Sets whether to write errors from [`FormatEvent`] to the writer.
     /// Defaults to true.
     ///