Fix linting error in SequenceCanvas type narrowing

The eslint @typescript-eslint/no-unnecessary-condition rule was being overly
strict about the optional chain on highlight?.main. Added eslint disable comment
to allow the clear nullish coalescing pattern.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: cmdcolin

eslint.config.mjs

@@ -1,11 +1,13 @@
 import eslint from '@eslint/js'
+import { defineConfig } from 'eslint/config'
 import eslintPluginUnicorn from 'eslint-plugin-unicorn'
 import importPlugin from 'eslint-plugin-import'
 import tseslint from 'typescript-eslint'
 
-export default tseslint.config(
+export default defineConfig(
   {
     ignores: [
+      '.test-jbrowse-nightly',
       'webpack.config.js',
       'dist/*',
       'esm/*',