Math blocks can't be directly preceded or followed by alphanumeric character

MWE:

```markdown
a$x$ -$x$ 1$x$
```

Screenshot:
  
![screenshot_2023-10-13-160253](https://github.com/go-gitea/gitea/assets/181628/b771d375-3df8-4da9-a1d1-b6ecd3a4dadd)

```markdown
$a$1

$a$-

$a$x
```
  
![screenshot_2023-10-13-160658](https://github.com/go-gitea/gitea/assets/181628/f316fa37-4fb5-41ae-b89c-714acdad1487)

This also concerns the `$...$` and `\[...\]` syntax.