[New+] Variable expansions inside template files

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### Description of the new feature / enhancement

Allow opt-in (for example per file extension) expansion of date/time and system variables also _inside_ text files. I have implemented that in PowerToys New+ compatible functionality implementation in Total Commander TCNewPlus plugin: https://github.com/michal-ruzicka/TCNewPlus#variable-expansion-inside-file-contents

### Scenario when this would be used?

For template of of meeting minutes in Markdown format, fill in date and time and attendees also inside of the file, not only in the filename: https://github.com/michal-ruzicka/TCNewPlus/blob/main/templates/080%20%24YYYY-%24MM-%24DD%20Meeting%20Notes.md

### Supporting information

It is implemented and tested in my Total Commander TCNewPlus plugin: https://github.com/michal-ruzicka/TCNewPlus

Start by locating PowerToys New+ and its existing filename variable-expansion entry point, then compare it with the linked TCNewPlus implementation and the Markdown meeting-notes template. Define how opt-in behavior is selected per file extension and how date, time, and system variables are expanded in file contents; done means the behavior is covered for the supported template scenario without changing non-opted-in files.

Written by the indexing model from the issue text.