Surface `addtextextension` setting in `TemplateVerifierOptions`

[JanKrivanek]

Is your feature request related to a problem? Please describe.

TemplateVerifier performs binary stream comparison of files with unrecognized or no extensions. Textual comparison is preferable in cases where content is text (due to normalization of newlines, byte order marks, etc.)

Describe the solution you'd like.

There is an option in Verify to tag file extension as textual: https://github.com/VerifyTests/EmptyFiles#addtextextension, the content verification then will work as on text (same as for well recognized textual extensions)

This should be surfaced via TemplateVerifierOptions (e.g. as AdditionalTextExtensions) and then passed to Verify.

Additionally the special case noextension extension should be allways implicitly added

[JanKrivanek]

More involved and 'unattended' solution might be to check if the extension is reconginzed as textual, if not attempt to autodetect the format of the file (more on that e.g. here: #5789 (comment)) and then call the AddExtension automatically if needed.