[templates] Fix applicationId template option

[jknaudt21]

Description of Change

We prevent the templating engine from matching the AppID to the App's name (the sourceName). This allows the applicationId parameter to work as expected when doing dotnet new -ap com.my.name. I also make sure that if we're using the (lowercased) app name as an AppID in case no parameter is passed.

The changes required some tricks using the templating engine. Mainly:

• Creating a separate variable (symbol) for the lowercased app name
• Creating a new symbol defaultAppId that is the concatenation of "com.company." with the lowercased app name
• Use the coalescing generator to evaluate the finalAppID. The result depends on whether the applicationId parameter was passed by the user. If the applicationId is null (i.e not given), we use the defaultAppId; otherwise we use whatever the user provided.

Issues Fixed

fixes #13297

[Eilon]

@jknaudt21 very nice! Can you confirm what happens with:

1. dotnet new maui
2. dotnet new maui -o FooBar_Baz.Banana

I want to make sure that those cases are also handled reasonably well. Or will they just get the default value com.companyname.mauiapp unless it's explicitly set at project creation time?

[jknaudt21]

I'm looking into seeing if something can be done with a symbol generator. https://github.com/dotnet/templating/wiki/Available-Symbols-Generators.

Marking PR as a draft until I arrive at a conclusion.

Edit: a solution was found that is the best of both worlds :)

[Eilon]

Amazing!