Enable .pdb files for Release config & add AzDO build script

[jonpryor]

Context: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1230070
Context: https://github.com/xamarin/XamarinComponents/blob/af66a9ea9536116b1cdcdd4bcc499f5fcb7232ea/.ci/build.yml

Current Microsoft practice requires the presence of symbol files
somewhere in order to provide useful stack traces in telemetry/etc.
The easiest way to have the symbol files is to (1) create them, and
(2) redistribute them within NuGet packages (so that e.g. the
Xamarin.Android SDK can in turn redistribute the .pdb files...)

Update LineEditor.csproj so that $(DebugType) is not cleared in
the Release configuration. This satisfies (1): create the symbol
files, so that they exist.

Update LineEditor.csproj to remove the @(PackageReference) to
NuGet.Build.Packaging and the @(PackageFile) item group, and
instead require the use of the msbuild /t:Pack command, which does
things mostly right.

Where msbuild /t:Pack falls down is that LineEditor.pdb isn't
included into the .nupkg file, which is something @jonpryor desires,
to make it easier to include LineEditor.pdb into the Xamarin.Android
.pdb/.vsix installers.

To cause LineEditor.pdb to be included in the .nupkg file,
override the $(AllowedOutputExtensionsInPackageBuildOutputFolder)
property to include .pdb.

Finally, add .ci/azure-pipelines.yml, which is a YAML build script
for Azure Pipelines. azure-pipelines.yml uses the .ci/build.yml
file from xamarin/XamarinComponents for signing and publishing.

[mattleibow]

Looking good so far.

[mattleibow]

We can remove all this (and the NuGet.Build.Packaging) package (lines 25-35). Then we can just do msbuild /t:pack and let the tools do the work.

[mattleibow]

Suggested change

	validPackagePrefixes:
	- Mono
	skipValidation: true

[mattleibow]

Add this to prevent any issues with an unexpected symbols package. Most wildcards will be able to skin the snupkg.