Updated common types XSD to remove errors from redefining Include attributes

[glektarssza]

Context

Some XML LSPs (like Lemmix, the engine that powers the redhat-developer/vscode-xml VSCode extension) struggle with the GitHub XSDs because they redefine the Include attribute inside the XSDs in order to redefine the documentation comments per element type.

This PR addresses that by prohibiting the use of the attribute inside the common types XSD, thus allowing this redefinition to occur.

Changes Made

• Added a XSD redefine block to src/MSBuild/MSBuild/Microsoft.Build.CommonTypes.xsd to ensure Include blocks are not marked as already defined from outside the file.

Testing

1. Load the XSD into a validation engine.
2. See that no errors are produced as a result of Include blocks being redefined.

[SimaTian]

======================================================================
Downloading XSD files

[PASS] Files saved to C:\Users\TBARTO~1\AppData\Local\Temp\xsd-pr13284-test

======================================================================
Validator 1: Python lxml (strict, libxml2-based)

Original (main):
Total errors: 19
Include-related errors: 19
file:/C:/Users/tbartonek/AppData/Local/Temp/xsd-pr13284-test/original/Microsoft.Build.CommonTypes.xsd:14:0:ERROR:SCHEMASP:SCHEMAP_AG_PROPS_CORRECT: local complex type: Duplicate attribute use 'Include'.
file:/C:/Users/tbartonek/AppData/Local/Temp/xsd-pr13284-test/original/Microsoft.Build.CommonTypes.xsd:75:0:ERROR:SCHEMASP:SCHEMAP_AG_PROPS_CORRECT: local complex type: Duplicate attribute use 'Include'.
file:/C:/Users/tbartonek/AppData/Local/Temp/xsd-pr13284-test/original/Microsoft.Build.CommonTypes.xsd:101:0:ERROR:SCHEMASP:SCHEMAP_AG_PROPS_CORRECT: local complex type: Duplicate attribute use 'Include'.
... and 16 more

PR branch:
Total errors: 0
Include-related errors: 0

[PASS] Sample .csproj validates against PR schema

======================================================================
Validator 2: .NET System.Xml.Schema (lenient)

Original (main):
Total errors: 0
Include-related errors: 0

PR branch:
Total errors: 0
Include-related errors: 0

======================================================================
SUMMARY

lxml (strict):
Original: 19 errors (19 Include-related)
PR: 0 errors (0 Include-related)
[PASS] PR eliminates 19 Include-redefinition errors

.NET (lenient):
Original: 0 errors
PR: 0 errors
[PASS] PR schema is valid under .NET

That being said, LGTM