The `DocumentFormat.OpenXml.Office2010.Drawing.TextMath` should not inherit OpenXmlLeafElement

**Describe the bug**

The `DocumentFormat.OpenXml.Office2010.Drawing.TextMath` (`a14:m`) is inherit OpenXmlLeafElement now. But as the document says, the `DocumentFormat.OpenXml.Office2010.Drawing.TextMath` (`a14:m`) will contain the OfficeMath element.

The sample OpenXML document:

```xml



123







123





```

As the [MS-ODRAWXML]-181211 says:

> 2.3.3.41 CT_TextMath
>
> Target namespace: http://schemas.microsoft.com/office/drawing/2010/main
Referenced by: m
This type contains either a math zone or document-level math properties. Math zone content can be
either an inline math zone or a math paragraph.
Child Elements:
m:oMath: A CT_OMath ([ISO/IEC29500-1:2016] section 22.1.2.77) element that specifies an
instance of mathematical text.
The following W3C XML Schema ([XMLSCHEMA1/2] section 2.1) fragment specifies the contents of this
complex type.





See section 5.1 for the full W3C XML Schema ([XMLSCHEMA1/2] section 2.1).

And

```xml





```

**Screenshots**
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**To Reproduce**
None.

**Observed behavior**

We can not get the ChildElements from `DocumentFormat.OpenXml.Office2010.Drawing.TextMath`. And it will cause some unexpected behavior, such as https://github.com/dotnet/Open-XML-SDK/issues/1033

**Expected behavior**

The `DocumentFormat.OpenXml.Office2010.Drawing.TextMath` should inherit OpenXmlCompositeElement type.

**Desktop (please complete the following information):**
- OS: [e.g. Windows]
- Office version [e.g. 16.0.15427.20178]
- .NET Target: (e.g. .NET Framework, .NET Core, UWP, Xamarin...)
- DocumentFormat.OpenXml Version: (e.g. 2.11.0)

**Additional context**
Add any other context about the problem here.