`linguist-fallback-language` attribute

Currently, people using a newer language on Github either have to accept a total lack of support for the language, or use .gitattributes with `linguist-language` to select a syntax highlighter that's "good enough". However, what if the language is then added to Linguist? Now the .gitattributes that have accumulated would override the actual, correct detection. Therefore, I suggest a `linguist-fallback-language` attribute that would allow Linguist to still classify the file if it does know the language, but provides a fallback in case it's not known.

The payload names no files or tests. Start by locating how .gitattributes and the existing linguist-language attribute are parsed and applied, then define coverage for known-language precedence and unknown-language fallback. Done means the fallback is used only when Linguist cannot identify the language, without overriding native detection.

Written by the indexing model from the issue text.