feat: make Termshow a standalone Quarto extension

[rich-iannone]

This PR refactors how the Termshow terminal recording player is integrated and distributed, making it available as a standalone Quarto extension. The changes remove manual asset management from the core codebase and leverage Quarto's extension and dependency system for easier use both inside and outside Great Docs. The documentation is also updated to guide users on using Termshow independently.

cc: @rpodcast , I've provided an extra section in the 'Terminal Recordings' section of the User Guide that walks through using Termshow in Quarto

[github-actions]

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