Expose Writer::new as public

Solves tokio-rs#2512

I'm writing a custom formatting layer and want to use UtcTime and format_time to format the time into a string.

tracing-subscriber/src/fmt/format/mod.rs

@@ -419,11 +419,7 @@ pub struct Format<F = Full, T = SystemTime> {
 // === impl Writer ===
 
 impl<'writer> Writer<'writer> {
-    // TODO(eliza): consider making this a public API?
-    // We may not want to do that if we choose to expose specialized
-    // constructors instead (e.g. `from_string` that stores whether the string
-    // is empty...?)
-    pub(crate) fn new(writer: &'writer mut impl fmt::Write) -> Self {
+    pub fn new(writer: &'writer mut impl fmt::Write) -> Self {
         Self {
             writer: writer as &mut dyn fmt::Write,
             is_ansi: false,