feature: DefaultViewLocator now public for users to override ViewModelToViewFunc

[fdoyon]

What kind of change does this PR introduce?
This is a feature update that allows users to override DefaultViewLocator.ViewModelToViewFunc if they are using custom naming conventions.

What is the current behavior?
Currently the users have to copy/paste the source of DefaultViewLocator and then change their ViewModelToViewFunc implementation. This is dangerous because if the logic of DefaultViewLocator changes, the copy-pasted code won't be able to carry the feature.

What is the new behavior?
Allow users to get a reference to the locator thusly:

var viewLocator = Locator.Current.GetService<IViewLocator>() as DefaultViewLocator;
viewLocator.ViewModelToViewFunc = x => x.Replace("ViewModel", "MyViewModel");

What might this PR break?
nothing

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
Discussed with @anaisbetts on slack.

[dnfclas]

All CLA requirements met.

[fdoyon]

For some reason an unrelated test is broken in the CI build: https://dev.azure.com/dotnet/ReactiveUI/_build/results?buildId=25723&view=logs&j=2d2b3007-3c5c-5840-9bb0-2b1ea49925f3&t=a8035825-ecc2-53a4-dc8f-13b782b0dc20&l=418

[RLittlesII]

This isn't an unrelated test. These tests verify changes to the public API of the code. You changing from internal to public changes the API surface so the test failed. The api approval change should be uploaded to the artifacts of the build so you can see the difference between the old API surface and the new API surface. You should also be able to this to reproduce this locally running .\build.ps1 --t=Test

[glennawatson]

Install a diff program like beyondcompare and it should be easy to modify with the API changes.

[glennawatson]

You can copy .received.txt to .approved.txt -- that might get around your current issues.

[fdoyon]

Thanks - now the winforms tests are crashing - I don't have a windows machine to test on I am afraid.

[glennawatson]

We'll do the rest for you. Thanks for getting it this far.

[codecov]

Codecov Report

Merging #2275 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2275   +/-   ##
=======================================
  Coverage   55.32%   55.32%           
=======================================
  Files         113      113           
  Lines        4334     4334           
  Branches      660      660           
=======================================
  Hits         2398     2398           
  Misses       1771     1771           
  Partials      165      165

Impacted Files	Coverage Δ
src/ReactiveUI/View/DefaultViewLocator.cs	96.61% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0dd8f2c...46884e3. Read the comment docs.

[RLittlesII]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).