Add Pager commands and syntax highlighting

Introduce a new pager command set (src/commands/pager.ts and built out files) with completion items and inline Markdown docs, and register PAGER_COMMANDS in the extension command groups. Update the grammar (duckyscript.tmLanguage.json) to include a pagerCommands pattern and add theme entries for the pager scope in both dark and light themes.

Author: aleff-github