For some syntax there is an option e.g. Developer -> New Syntax from Plain Text.tmLanguage... but on some files this option is not listed under tools.
For example for a JSON file with scope source.json.sublime.project there is no New Syntax from … option.
I have PackageDev Plug-in installed, which I believe adds this option. Why does it show up for some sources but not for others?
It’s only used for converting a .tmLanguage file to its .sublime-syntax equivalent iirc. I haven’t played with .tmLanguage for quite a long time.
Also, this menu item is provided by ST natively (through a Default plugin). PackageDev provides the syntax definition for project files, however.
This entry is only present for .tmLanguage Plist files, not JSON or YAML equivalents. Here are some documented steps on how to convert any .tmLanguage, .tmLanguage.json, tmLanguage.yaml, and .tmLanguage.plist to a .sublime-text YAML format: https://github.com/sourcegraph/syntect_server/blob/master/README.md#adding-languages