Enforce .bicep extension on Bicep build command

[jeskew]

Fixes #1369
Fixes #5091

This PR allows bicep build to exit early with a meaningful error message when a non-bicep file is provided as the command's entry point. It does so by enforcing that bicep files use the .bicep extension (per the discussions in #5091 and in #1369 (comment)) by adding a new AuxiliaryTextFile subtype of ISourceFile to use for any file that is not a JSON ARM template or file with the .bicep extension. This source file type has a corresponding empty semantic model.

This is a breaking change, and it might make sense to bundle this with other breaking changes in a release with a meaningful version number bump. I did have to modify a number of tests that were creating bicep files without the correct extension, and I suspect many users who are synthesizing bicep files and then invoking the compiler may need to do the same.

Contributing a feature

• I have opened a new issue for the proposal, or commented on an existing one, and ensured that the Bicep maintainers are good with the design of the feature being implemented
• I have included "Fixes #{issue_number}" in the PR description, so GitHub can link to the issue and close it when the PR is merged
• I have appropriate test coverage of my new feature

[shenglol]