fix(parser): comments in subexpressions of let/mut (#15375)

Closes #15305

# Description

Basically turns off `skip_comments` of the lex function for right hand
side expressions of `let`/`mut`, just as in `parse_const`.

# User-Facing Changes

Should be none.

# Tests + Formatting

+1

# After Submitting

crates/nu-parser/src/parse_keywords.rs

@@ -3331,7 +3331,7 @@ pub fn parse_let(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipeline
                         spans[span.0 + 1].start,
                         &[],
                         &[],
-                        true,
+                        false,
                     );
 
                     if let Some(parse_error) = parse_error {
@@ -3613,7 +3613,7 @@ pub fn parse_mut(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipeline
                         spans[span.0 + 1].start,
                         &[],
                         &[],
-                        true,
+                        false,
                     );
 
                     if let Some(parse_error) = parse_error {

crates/nu-parser/src/parser.rs

@@ -5208,7 +5208,7 @@ pub fn parse_assignment_expression(
         rhs_span.start,
         &[],
         &[],
-        true,
+        false,
     );
     working_set.parse_errors.extend(rhs_error);