Update rustfmt.toml to support nightly flags

In LDK Node we found a `rustfmt.toml` that enables a few nice `nightly`
features (such as automatic import grouping) while being backwards
compatible with `rustfmt` stable. That means that in day-to-day we can
continue to just do `cargo fmt`, but then have a simple weekly `rustfmt`
job that cleans up the codebase a bit more.

Author: tnull

rustfmt.toml

@@ -4,9 +4,16 @@ hard_tabs = true
 use_field_init_shorthand = true
 max_width = 100
 match_block_trailing_comma = true
-# UNSTABLE: format_code_in_doc_comments = true
-# UNSTABLE: overflow_delimited_expr = true
-# UNSTABLE: comment_width = 100
-# UNSTABLE: format_macro_matchers = true
-# UNSTABLE: format_strings = true
-# UNSTABLE: group_imports = "StdExternalCrate"
+format_code_in_doc_comments = true
+comment_width = 100
+format_macro_matchers = true
+group_imports = "StdExternalCrate"
+reorder_imports = true
+imports_granularity = "Module"
+normalize_comments = true
+normalize_doc_attributes = true
+style_edition = "2021"
+# TBD: do we want comment and string wrapping?
+#wrap_comments = true
+#format_strings = true
+#overflow_delimited_expr = true