fix: use prefixFor for BoolWithInverseFlag names and aliases

BoolWithInverseFlag.String picked - or -- with len(name) == 1, which counts bytes, while the rest of the library uses prefixFor, which counts runes. a one-character non-ascii name or alias was shown as --ü in its help and as -ü for any other flag.

flag_bool_with_inverse_test.go

@@ -367,6 +367,12 @@ func TestBoolWithInverseString(t *testing.T) {
 			required: true,
 			expected: "-[no-]e\t",
 		},
+		{
+			testName: "single-char multibyte flag name",
+			flagName: "ü",
+			required: true,
+			expected: "-[no-]ü\t",
+		},
 		{
 			testName: "multi-char flag name",
 			flagName: "env",
@@ -412,6 +418,13 @@ func TestBoolWithInverseString(t *testing.T) {
 			aliases:  []string{"c"},
 			expected: "--[no-]color, -c\t(default: false)",
 		},
+		{
+			testName: "short multibyte alias",
+			flagName: "color",
+			required: false,
+			aliases:  []string{"ü"},
+			expected: "--[no-]color, -ü\t(default: false)",
+		},
 		{
 			testName: "long alias",
 			flagName: "color",