Preserve existing package source mappings in XSAddNugetSource.prgx

[Copilot]

The script was unconditionally adding/overwriting packageSourceMapping entries on every run, causing pre-existing third-party source mappings to be lost.

Changes

Main body — conditional mapping logic

• If packageSourceMapping section already exists: only update/add the XSharp.* → XSharp Offline Packages entry; all other mappings are left untouched
• If no packageSourceMapping section exists: create it fresh with both * → nuget.org and XSharp.* → XSharp Offline Packages (adding nuget.org as a package source too if absent)

GetSetSourceMapping — true update semantics

• Previously skipped entirely if the source key was already mapped, even if the required pattern was missing
• Now locates the existing <packageSource> element and adds the pattern if absent — making the function a genuine "update or add"

<!-- Before (existing mapping silently dropped if source key was present) -->
<packageSourceMapping>
  <packageSource key="XSharp Offline Packages" />  <!-- no pattern added -->
</packageSourceMapping>

<!-- After -->
<packageSourceMapping>
  <packageSource key="XSharp Offline Packages">
    <package pattern="XSharp.*" />   <!-- ensured present -->
  </packageSource>
  <!-- all other pre-existing entries preserved -->
</packageSourceMapping>