Use StringBuilder in BufferLine.translateToString

Replace O(n²) string concatenation in the translate loop with
StringBuilder.append, matching the pattern used elsewhere in the project.

Fixes #5939

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/common/buffer/BufferLine.ts

@@ -8,6 +8,7 @@ import { AttributeData } from 'common/buffer/AttributeData';
 import { CellData } from 'common/buffer/CellData';
 import { Attributes, BgFlags, CHAR_DATA_ATTR_INDEX, CHAR_DATA_CHAR_INDEX, CHAR_DATA_WIDTH_INDEX, Content, NULL_CELL_CHAR, NULL_CELL_CODE, NULL_CELL_WIDTH, WHITESPACE_CELL_CHAR } from 'common/buffer/Constants';
 import { stringFromCodePoint } from 'common/input/TextDecoder';
+import { StringBuilder } from 'common/StringBuilder';
 
 // Buffer memory layout:
 //
@@ -570,12 +571,12 @@ export class BufferLine implements IBufferLine {
     if (outColumns) {
       outColumns.length = 0;
     }
-    let result = '';
+    const resultBuilder = new StringBuilder();
     while (startCol < endCol) {
       const content = this._data[startCol * Constants.CELL_INDICIES + Cell.CONTENT];
       const cp = content & Content.CODEPOINT_MASK;
       const chars = (content & Content.IS_COMBINED_MASK) ? this._combined[startCol] : (cp) ? stringFromCodePoint(cp) : WHITESPACE_CELL_CHAR;
-      result += chars;
+      resultBuilder.append(chars);
       if (outColumns) {
         for (let i = 0; i < chars.length; ++i) {
           outColumns.push(startCol);
@@ -586,6 +587,7 @@ export class BufferLine implements IBufferLine {
     if (outColumns) {
       outColumns.push(startCol);
     }
+    const result = resultBuilder.toString();
     if (isCanonicalRequest) {
       const cacheEntry = this._getStringCacheEntry(true)!;
       cacheEntry.value = result;