fix: exclude generator from coverage, fix go:generate path

- Add //go:build ignore to generator (build tool, not runtime code)
- Point go:generate at main.go file directly (required with build tag)
- Coverage: 89.3% → 93.0%

Author: kehoecj

internal/generate/knownfiles/main.go

@@ -1,3 +1,5 @@
+//go:build ignore
+
 // Command knownfiles generates known_files_gen.go from GitHub Linguist's languages.yml.
 //
 // It maps Linguist language names to config-file-validator file types and extracts

pkg/filetype/file_type.go

@@ -1,4 +1,4 @@
-//go:generate go run ../../internal/generate/knownfiles
+//go:generate go run ../../internal/generate/knownfiles/main.go
 
 package filetype