parser: dont allow single letter enums (#6526)

vlib/v/checker/tests/enum_single_letter.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/enum_single_letter.vv:1:6: error: single letter capital names are reserved for generic template types.
+    1 | enum E {
+      |      ^
+    2 |     v w
+    3 | }

vlib/v/checker/tests/enum_single_letter.vv

@@ -0,0 +1,4 @@
+enum E {
+    v w
+}
+println(E.v)

vlib/v/parser/parse_type.v

@@ -206,7 +206,7 @@ pub fn (mut p Parser) parse_any_type(language table.Language, is_ptr, check_dot
 		}
 	} else if p.expr_mod != '' {
 		name = p.expr_mod + '.' + name
-	} else if p.mod != 'builtin' && name !in p.table.type_idxs && name.len > 1 {
+	} else if p.mod != 'builtin' && name.len > 1 && name !in p.table.type_idxs {
 		// `Foo` in module `mod` means `mod.Foo`
 		name = p.mod + '.' + name
 	}

vlib/v/parser/parser.v

@@ -1723,6 +1723,10 @@ fn (mut p Parser) enum_decl() ast.EnumDecl {
 	p.check(.key_enum)
 	end_pos := p.tok.position()
 	enum_name := p.check_name()
+	if enum_name.len == 1 {
+		p.error_with_pos('single letter capital names are reserved for generic template types.',
+			end_pos)
+	}
 	name := p.prepend_mod(enum_name)
 	p.check(.lcbr)
 	enum_decl_comments := p.eat_comments()