Add from-eml command (#1656)

* from-eml initial ver

* Adding tests for `from-eml`

* Add eml to prepares_and_decorates_filesystem_source_files

* Sort the file order

Co-authored-by: Jonathan Turner <jonathandturner@users.noreply.github.com>

crates/nu-cli/tests/format_conversions/mod.rs

@@ -1,5 +1,6 @@
 mod bson;
 mod csv;
+mod eml;
 mod html;
 mod ics;
 mod json;