fix unknown_command when parsing certain strings with equal signs (#14053)

# Description

Prevents errors when `=` is used before the end of:

- strings in lists/records (with a symbol adjacent to the quotes)
- raw strings

```
> ["=a"]
Error: nu::parser::unknown_command

  × Unknown command.
   ╭─[entry #9:1:1]
 1 │ ["=a"]
   · ───┬──
   ·    ╰── unknown command
   ╰────
```

```
> r#'=a'#
Error: nu::parser::unknown_command

  × Unknown command.
   ╭─[entry #5:1:1]
 1 │ r#'=a'#
   · ───┬───
   ·    ╰── unknown command
   ╰────
```

Closes #13902, closes #13901, closes #9879, closes #6401, closes #5806

# User-Facing Changes

Variable names in environment shorthand assignments must satisfy
`is_identifier`.

crates/nu-parser/src/parser.rs

@@ -2943,6 +2943,10 @@ pub fn parse_string_strict(working_set: &mut StateWorkingSet, span: Span) -> Exp
             working_set.error(ParseError::Unclosed("\'".into(), span));
             return garbage(working_set, span);
         }
+        if bytes.starts_with(b"r#") && (bytes.len() == 1 || !bytes.ends_with(b"#")) {
+            working_set.error(ParseError::Unclosed("r#".into(), span));
+            return garbage(working_set, span);
+        }
     }
 
     let (bytes, quoted) = if (bytes.starts_with(b"\"") && bytes.ends_with(b"\"") && bytes.len() > 1)
@@ -5396,10 +5400,12 @@ pub fn parse_expression(working_set: &mut StateWorkingSet, spans: &[Span]) -> Ex
 
             let starting_error_count = working_set.parse_errors.len();
 
-            let lhs = parse_string_strict(
-                working_set,
-                Span::new(spans[pos].start, spans[pos].start + point - 1),
-            );
+            let lhs_span = Span::new(spans[pos].start, spans[pos].start + point - 1);
+            if !is_identifier(working_set.get_span_contents(lhs_span)) {
+                break;
+            }
+
+            let lhs = parse_string_strict(working_set, lhs_span);
             let rhs = if spans[pos].start + point < spans[pos].end {
                 let rhs_span = Span::new(spans[pos].start + point, spans[pos].end);