Adjust ResourceLoader(String) to match documents

[YexuanXiao]

In documents ResourceLoader and WindowsAppSDK Localize strings in your UI and the app package manifest, str in Microsoft.Windows.ApplicationModel.Resources.ResourceLoader.ResourceLoader(String str) represents resourceMap, usually the file name of resw. In the current implementation, str represents the file name of pri the str of the old obsolete API Windows.ApplicationModel.Resources.ResourceLoader(String str) represents the resourceMap.

So making the two more consistent seems to be a better solution, rather than using Microsoft.Windows.ApplicationModel.Resources.ResourceLoader.GetDefaultResourceFilePath().

documents:

ResourceLoader("ErrorMessages") // implicit "resources.pri" or "modulename.pri"

implement:

ResourceLoader(ResourceLoader.GetDefaultResourceFilePath(), "ErrorMessages")

this pr:
same as documents

breaking change

before:

ResourceLoader("filename.pri") // implicit "Resources" subtree

after:

ResourceLoader("filename.pri", "Resources")

Original issue: #4312 #2406

[lauren-ciha]

Thank you for your contribution! In this case, the docs need an update. Here is the PR for that change.