Create Spec for normalizing validation of settings between nuget.config & VS Options

[donnie-msft]

Create a spec to make progress on #12245

Examples of topics to address:

• Malformed config
  • Sometimes causes odd behavior when opening VS Options
  • Sometimes quietly ignores the invalid change (eg, `<
• Inconsistencies between hand-editing and using VS Options to modify settings.
  • VS Options enforces distinct Package Source names (case insensitive)
    typos...

  <packageSourceMapping>
    <clear />
    <paadsfasdfckageSource key="nuget">
      <package pattern="jQuery" />

Pattern pattern (sneaky, looks right, but it's wrong and it's hard to tell in VS that it was ignored

<packageSourceMapping>
    <clear />
    <packageSource key="nuget">
      <pattern pattern="jQuery" />

• Hand-editing a nuget.config with both "nuget.org" and "NuGet.ORg" is accepted by VS (no errors).

Ideally, we'd have consistent behavior across the board, but the Spec needs to call out any breaking change potential.

/cc @erdembayar

[donnie-msft]

@albarry4 in Sprint planning, @aortiz-msft asked me to assign this to you to help define the experience. Thanks!
https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1699214