feat: add \xHH escape sequence support in basic strings

TOML v1.1.0 introduces the \xHH escape notation for basic strings,
allowing two-digit hex escapes for Unicode code points U+0000 to
U+00FF.

We keep emitting \u00XX for backwards compatibility.

unmarshaler_test.go

@@ -759,6 +759,62 @@ huey = 'dewey'
 				}
 			},
 		},
+		{
+			desc:  "basic string hex escape lowercase letter",
+			input: `A = "\x61"`,
+			gen: func() test {
+				type doc struct {
+					A string
+				}
+
+				return test{
+					target:   &doc{},
+					expected: &doc{A: "a"},
+				}
+			},
+		},
+		{
+			desc:  "basic string hex escape null byte",
+			input: `A = "\x00"`,
+			gen: func() test {
+				type doc struct {
+					A string
+				}
+
+				return test{
+					target:   &doc{},
+					expected: &doc{A: "\x00"},
+				}
+			},
+		},
+		{
+			desc:  "basic string hex escape max value",
+			input: `A = "\xFF"`,
+			gen: func() test {
+				type doc struct {
+					A string
+				}
+
+				return test{
+					target:   &doc{},
+					expected: &doc{A: "\u00FF"},
+				}
+			},
+		},
+		{
+			desc: "multiline basic string hex escape",
+			input: `A = """\x61"""`,
+			gen: func() test {
+				type doc struct {
+					A string
+				}
+
+				return test{
+					target:   &doc{},
+					expected: &doc{A: "a"},
+				}
+			},
+		},
 		{
 			desc:  "spaces around dotted keys",
 			input: "a . b = 1",
@@ -3260,6 +3316,18 @@ world'`,
 			desc: `invalid escape char basic multiline string`,
 			data: `A = """\z"""`,
 		},
+		{
+			desc: `invalid hex escape non-hex character in basic string`,
+			data: `A = "\xGG"`,
+		},
+		{
+			desc: `incomplete hex escape in basic string`,
+			data: `A = "\x6"`,
+		},
+		{
+			desc: `invalid hex escape non-hex character in multiline basic string`,
+			data: `A = """\xGG"""`,
+		},
 		{
 			desc: `invalid inf`,
 			data: `A = ick`,

unstable/parser.go

@@ -785,6 +785,13 @@ func (p *Parser) parseMultilineBasicString(b []byte) ([]byte, []byte, []byte, er
 				builder.WriteByte('\t')
 			case 'e':
 				builder.WriteByte(0x1B)
+			case 'x':
+				x, err := hexToRune(atmost(token[i+1:], 2), 2)
+				if err != nil {
+					return nil, nil, nil, err
+				}
+				builder.WriteRune(x)
+				i += 2
 			case 'u':
 				x, err := hexToRune(atmost(token[i+1:], 4), 4)
 				if err != nil {
@@ -944,6 +951,13 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 				builder.WriteByte('\t')
 			case 'e':
 				builder.WriteByte(0x1B)
+			case 'x':
+				x, err := hexToRune(token[i+1:len(token)-1], 2)
+				if err != nil {
+					return nil, nil, nil, err
+				}
+				builder.WriteRune(x)
+				i += 2
 			case 'u':
 				x, err := hexToRune(token[i+1:len(token)-1], 4)
 				if err != nil {