Add IsExternalInit for relevant TFM's

[ChrisPulman]

What kind of change does this PR introduce?

Update

What is the current behavior?

IsExternalInit is required for older TFM's

What is the new behavior?

IsExternalInit is generated

What might this PR break?

None

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:

[fubar-coder]

@ChrisPulman Hi, this PR seems incomplete/wrong, because it prevents usage of source packages like Polyfill due to duplicate definition of IsExternalInit.

There are some solutions:

1. Never add IsExternalInit and let the developer choose the IsExternalInit implementation (and maybe add a FAQ and issue (closed) for the resulting error message
2. Don't emit IsExternalInit and just use get; set; instead of get; init;
3. Guard the implementation with a #if !REACTIVEUI_NO_ISEXTERNALINIT or something like that
4. Allow setting a configuration in the csproj file to deactivate emitting IsExternalInit, similar to what PolySharp does

[glennawatson]

@fubar-coder Added a new issue for tracking this. The first option is likely the best, if the user needs it just include on of the several frameworks out there that adds support.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.