[feature] Detect language by filename

[ianlewis]

Currently linguist only supports detecting a single language by filename even though sometimes there could be multiple matches.

Use linguist to detect the language by file name and if there is only one match use that. Otherwise determine the language based on the contents.

Related #513
Related dayvonjersen/linguist#11

[github-actions]

There are TODOs referencing this issue:

1. internal/scanner/scanner.go:140: Use linguist to detect by file name