Allow viewing the source code of blocks (#894)

* Add spans to blocks and view command

* Better description; Cleanup

* Rename "view" command to "view-source"

crates/nu-parser/src/parse_keywords.rs

@@ -761,7 +761,7 @@ pub fn parse_module_block(
         }
     }
 
-    let mut overlay = Overlay::new();
+    let mut overlay = Overlay::from_span(span);
 
     let block: Block = output
         .block

crates/nu-parser/src/parser.rs

@@ -2956,6 +2956,7 @@ pub fn parse_block_expression(
     let captures = find_captures_in_block(working_set, &output, &mut seen, &mut seen_decls);
 
     output.captures = captures;
+    output.span = Some(span);
 
     working_set.exit_scope();