fix: paste th should as a line like td

[zzxming]

Description

when there are consecutive 'th' in the pasted table, the pasted 'th' will be merged.

I checked the source code and found that 'th' was not parsed as a line like 'td', so this situation occurred

Reproduction

paste the table below

paste html

<table>
  <tbody>
    <tr>
      <th scope="col">Person</th>
      <th scope="col">Most interest in</th>
      <th scope="col">Age</th>
    </tr>
    <tr>
      <th scope="row">Dennis</th>
      <td>Web accessibility</td>
      <td>45</td>
    </tr>
    <tr>
      <th scope="row">Sarah</th>
      <td>JavaScript frameworks</td>
      <td>29</td>
    </tr>
  </tbody>
</table>

Current Behavior

and the table delta like this

[
    {
        "insert": "PersonMost interest inAge\n",
        "attributes": {
            "table": 1
        }
    },
    {
        "insert": "Dennis\nWeb accessibility\n45\n",
        "attributes": {
            "table": 2
        }
    },
    {
        "insert": "Sarah\nJavaScript frameworks\n29\n",
        "attributes": {
            "table": 3
        }
    }
]

Expected Behavior

the correct table delta should be like this

[
    {
        "insert": "Person\nMost interest in\nAge\n",
        "attributes": {
            "table": 1
        }
    },
    {
        "insert": "Dennis\nWeb accessibility\n45\n",
        "attributes": {
            "table": 2
        }
    },
    {
        "insert": "Sarah\nJavaScript frameworks\n29\n",
        "attributes": {
            "table": 3
        }
    }
]

Additional comments

I have my own quill table module plugin that provides many table related operations. but I found this issue when I paste th tag into editor. there is a lot of code related to 'isLine' that has not been exposed to the outside, so I am unable to hack it.