Add into duration (#4683)

* Add `into duration` command

* Avoid using unwrap()

* Use existing logic to parse duration strings

crates/nu-parser/src/lib.rs

@@ -14,7 +14,8 @@ pub use lex::{lex, Token, TokenContents};
 pub use lite_parse::{lite_parse, LiteBlock};
 
 pub use parser::{
-    is_math_expression_like, parse, parse_block, parse_external_call, trim_quotes, Import,
+    is_math_expression_like, parse, parse_block, parse_duration_bytes, parse_external_call,
+    trim_quotes, Import,
 };
 
 #[cfg(feature = "plugin")]