refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/browser/Terminal.test.ts

@@ -1332,8 +1332,8 @@ describe('Terminal', () => {
           (!(i % 3))
             ? input[i]
             : (i % 3 === 1)
-              ? input.substr(i, 2)
-              : input.substr(i - 1, 2),
+              ? input.slice(i, i + 2)
+              : input.slice(i - 1, i + 1),
           terminal.buffer.lines.get(bufferIndex[0])!.loadCell(bufferIndex[1], new CellData()).getChars());
       }
     });

src/browser/renderer/CustomGlyphs.ts

@@ -414,10 +414,10 @@ function drawPatternChar(
     let b: number;
     let a: number;
     if (fillStyle.startsWith('#')) {
-      r = parseInt(fillStyle.substr(1, 2), 16);
-      g = parseInt(fillStyle.substr(3, 2), 16);
-      b = parseInt(fillStyle.substr(5, 2), 16);
-      a = fillStyle.length > 7 && parseInt(fillStyle.substr(7, 2), 16) || 1;
+      r = parseInt(fillStyle.slice(1, 3), 16);
+      g = parseInt(fillStyle.slice(3, 5), 16);
+      b = parseInt(fillStyle.slice(5, 7), 16);
+      a = fillStyle.length > 7 && parseInt(fillStyle.slice(7, 9), 16) || 1;
     } else if (fillStyle.startsWith('rgba')) {
       ([r, g, b, a] = fillStyle.substring(5, fillStyle.length - 1).split(',').map(e => parseFloat(e)));
     } else {