Add more eslint rules

Found in https://github.com/refined-github/refined-github/blob/65aab3908a1cce9d6b9de95e3fb3de1060d47649/eslint-rules/restricted-syntax.js

## Prefer selectors as array

```js
'byo/selectors-array-for-complex-strings': ['error', {
selector:
':matches([callee.name=delegate], [callee.name=$], [callee.name=$$], [callee.name=$optional], [callee.name=closestElement], [callee.name=closestElementOptional], [callee.name=observe], [callee.property.name=querySelector], [callee.property.name=querySelectorAll])[arguments.0.value=/,/][arguments.0.value.length>=20]:not([arguments.0.value=/:has|:is|:not/])',
message: 'Instead of a single string, pass an array of selectors and add comments to each selector',
}],
'byo/selectors-string-for-single-array-item': ['error', {
selector:
':matches([callee.name=delegate], [callee.name=$], [callee.name=$$], [callee.name=$optional], [callee.name=closestElement], [callee.name=closestElementOptional], [callee.name=observe], [callee.property.name=querySelector], [callee.property.name=querySelectorAll])[arguments.0.type=ArrayExpression][arguments.0.elements.length=1]:not([arguments.0.value=/:has|:is/])',
message: "If it's a single selector, use a single string instead of an array",
}],

```

## Null/expected guidance

```js
'byo/no-non-null-optional': ['error', {
selector: 'TSNonNullExpression > CallExpression > [name=$optional]',
message: 'Use `$()` instead of non-null `$optional()`. Use it as `import {expectElement as $}`',
}],
'byo/no-non-null-expect-element': ['error', {
selector: 'TSNonNullExpression > CallExpression > [name=$]',
message: 'Unused null expression: !',
}],
'byo/no-non-null-closest': ['error', {
selector: 'TSNonNullExpression > CallExpression > [name=closestElement]',
message: 'Unused null expression: ! — closestElement() already throws when the element is not found',
}],
```

## Use helpers

```js
'byo/prefer-element-exists': ['error', {
message: 'Use `elementExists` for checking if an element exists',
selector: '*[test.type="CallExpression"][test.callee.name="$optional"],'
+ '*[test.type="UnaryExpression"][test.operator="!"][test.argument.type="CallExpression"][test.argument.callee.name="$optional"]',
}],
```

Start with eslint-rules/restricted-syntax.js at the referenced commit and inspect how existing byo rules are registered. Add the listed selector-array, null/expected, and element-existence rules there, then verify that the ESLint configuration recognizes their patterns and messages.

Written by the indexing model from the issue text.