Add space-unary-ops lint rule and fix SixelHandler violation

Enable @stylistic/space-unary-ops to warn on spaces after unary
operators like ! in conditions. Fix existing violation in SixelHandler.

Fixes #5898

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

addons/addon-image/src/SixelHandler.ts

@@ -91,7 +91,7 @@ export class SixelHandler implements IDcsHandler, IResetHandler {
     const height = this._dec.height;
 
     // partial fix for https://github.com/jerch/xterm-addon-image/issues/37
-    if (!width || ! height) {
+    if (!width || !height) {
       if (height) {
         this._storage.advanceCursor(height);
       }

eslint.config.mjs

@@ -43,6 +43,7 @@ export default tseslint.config(
         singleline: { delimiter: 'comma', requireLast: false }
       }],
       '@stylistic/type-annotation-spacing': 'warn',
+      '@stylistic/space-unary-ops': 'warn',
 
       '@typescript-eslint/array-type': ['warn', { default: 'array', readonly: 'generic' }],
       '@typescript-eslint/consistent-type-assertions': 'warn',