Add '.' and '-' to restricted characters

This means that commands cannot start with these characters.
However, we get the following benefits:
* Negative numbers               > -10
* Ranges with negative numbers   > -10..-1
* Left-unbounded ranges          > ..10

crates/nu-parser/src/parser.rs

@@ -2103,7 +2103,7 @@ pub fn parse_expression(
 
     match bytes[0] {
         b'0' | b'1' | b'2' | b'3' | b'4' | b'5' | b'6' | b'7' | b'8' | b'9' | b'(' | b'{'
-        | b'[' | b'$' | b'"' | b'\'' => parse_math_expression(working_set, spans),
+        | b'[' | b'$' | b'"' | b'\'' | b'.' | b'-' => parse_math_expression(working_set, spans),
         _ => parse_call(working_set, spans, true),
     }
 }