feat: add help suggestion to disambiguate parsing order with intersection

Author: Aloso

pomsky-lib/src/diagnose/help.rs

@@ -175,6 +175,11 @@ pub(super) fn get_compiler_help(kind: &CompileErrorKind, _span: Span) -> Option<
                     .to_string(),
             )
         }
+        CompileErrorKind::BadIntersection => Some(
+            "One character sets can be intersected.\n\
+            Parentheses may be required to clarify the parsing order."
+                .to_string(),
+        ),
 
         _ => None,
     }

pomsky-lib/tests/testcases/intersections/e_multiple.txt

@@ -0,0 +1,7 @@
+#! expect=error
+[w] & [d] [s]
+-----
+ERROR: Intersecting these expressions is not supported. Only character sets can be intersected.
+HELP: One character sets can be intersected.
+Parentheses may be required to clarify the parsing order.
+SPAN: 0..13

pomsky-lib/tests/testcases/intersections/e_string_empty.txt

@@ -0,0 +1,7 @@
+#! expect=error
+'a' & ''
+-----
+ERROR: Intersecting these expressions is not supported. Only character sets can be intersected.
+HELP: One character sets can be intersected.
+Parentheses may be required to clarify the parsing order.
+SPAN: 0..8

pomsky-lib/tests/testcases/intersections/e_string_too_long.txt

@@ -0,0 +1,7 @@
+#! expect=error
+[w] & 'ab'
+-----
+ERROR: Intersecting these expressions is not supported. Only character sets can be intersected.
+HELP: One character sets can be intersected.
+Parentheses may be required to clarify the parsing order.
+SPAN: 0..10

pomsky-lib/tests/testcases/intersections/e_with_assertions.txt

@@ -0,0 +1,7 @@
+#! expect=error
+'a' & 'b' $
+-----
+ERROR: Intersecting these expressions is not supported. Only character sets can be intersected.
+HELP: One character sets can be intersected.
+Parentheses may be required to clarify the parsing order.
+SPAN: 0..11

pomsky-lib/tests/testcases/intersections/strings.txt

@@ -0,0 +1,3 @@
+'a' & 'b'
+-----
+[a&&b]