Add support for resource text files (*.restext)

**Is your feature request related to a problem? Please describe.**
Some developers use [resource text files](https://learn.microsoft.com/en-us/dotnet/core/extensions/create-resource-files#resources-in-text-files) instead of RESX. Resgen.exe supports this, but I'm not able to find any source code or API aside from the [StronglyTypedResourceBuilder class](https://learn.microsoft.com/en-us/dotnet/api/system.resources.tools.stronglytypedresourcebuilder?view=netframework-4.8).

**Describe the solution you'd like**
If \*.restext support doesn't already exist, we can add a TextResourceReader implementation of IResourceReader, then use this to add \*.restext support. In the process, we will have generalized the code to any IResourceReader, giving us future support for other formats like Portable Objects (\*.po).

**Example TextResourceReader.cs**
[TextResourceReader.cs.txt](https://github.com/dotnet/ResXResourceManager/files/9697506/TextResourceReader.cs.txt)