perf: use single loop for cellsNeeded sum in BufferReflow

Replace map().reduce() with one accumulator loop in
reflowSmallerGetNewLineLengths to avoid allocating an intermediate
array when summing wrapped line trimmed lengths.

Fixes #5942

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

src/common/buffer/BufferReflow.ts

@@ -178,7 +178,10 @@ export function reflowLargerApplyNewLayout(lines: CircularList<IBufferLine>, new
  */
 export function reflowSmallerGetNewLineLengths(wrappedLines: BufferLine[], oldCols: number, newCols: number): number[] {
   const newLineLengths: number[] = [];
-  const cellsNeeded = wrappedLines.map((l, i) => getWrappedLineTrimmedLength(wrappedLines, i, oldCols)).reduce((p, c) => p + c);
+  let cellsNeeded = 0;
+  for (let i = 0; i < wrappedLines.length; i++) {
+    cellsNeeded += getWrappedLineTrimmedLength(wrappedLines, i, oldCols);
+  }
 
   // Use srcCol and srcLine to find the new wrapping point, use that to get the cellsAvailable and
   // linesNeeded