json: count whole grapheme clusters as columns for position purposes

This means we can actually point at a column in the console without it
getting misaligned by multi-byte UTF-8 sequences and Unicode combining
characters.

zcl/json/scanner.go

@@ -3,6 +3,7 @@ package json
 import (
 	"fmt"
 
+	"github.com/apparentlymart/go-textseg/textseg"
 	"github.com/apparentlymart/go-zcl/zcl"
 )
 
@@ -213,15 +214,14 @@ Byte:
 		case b < 32:
 			break Byte
 		default:
-			// TODO: Use Unicode Text Segmentation spec to advance
-			// Column only once per grapheme cluster, rather than once per
-			// byte.
-			// Consume one or more UTF-8 codepoints that together form
-			// a single grapheme cluster.
+			// Advance by one grapheme cluster, so that we consider each
+			// grapheme to be a "column".
+			// Ignoring error because this scanner cannot produce errors.
+			advance, _, _ := textseg.ScanGraphemeClusters(buf[i:], true)
 
-			p.Pos.Byte++
+			p.Pos.Byte += advance
 			p.Pos.Column++
-			i++
+			i += advance
 
 			escaping = false
 		}

zcl/json/scanner_test.go

@@ -738,6 +738,78 @@ func TestScan(t *testing.T) {
 				},
 			},
 		},
+		{
+			`"🇬🇧"`,
+			[]token{
+				{
+					Type:  tokenString,
+					Bytes: []byte(`"🇬🇧"`),
+					Range: zcl.Range{
+						Start: zcl.Pos{
+							Byte:   0,
+							Line:   1,
+							Column: 1,
+						},
+						End: zcl.Pos{
+							Byte:   10,
+							Line:   1,
+							Column: 4,
+						},
+					},
+				},
+				{
+					Type: tokenEOF,
+					Range: zcl.Range{
+						Start: zcl.Pos{
+							Byte:   10,
+							Line:   1,
+							Column: 4,
+						},
+						End: zcl.Pos{
+							Byte:   10,
+							Line:   1,
+							Column: 4,
+						},
+					},
+				},
+			},
+		},
+		{
+			`"á́́́́́́́"`,
+			[]token{
+				{
+					Type:  tokenString,
+					Bytes: []byte(`"á́́́́́́́"`),
+					Range: zcl.Range{
+						Start: zcl.Pos{
+							Byte:   0,
+							Line:   1,
+							Column: 1,
+						},
+						End: zcl.Pos{
+							Byte:   19,
+							Line:   1,
+							Column: 4,
+						},
+					},
+				},
+				{
+					Type: tokenEOF,
+					Range: zcl.Range{
+						Start: zcl.Pos{
+							Byte:   19,
+							Line:   1,
+							Column: 4,
+						},
+						End: zcl.Pos{
+							Byte:   19,
+							Line:   1,
+							Column: 4,
+						},
+					},
+				},
+			},
+		},
 	}
 
 	for _, test := range tests {