Document how to create GitHub Copilot chat variables using tools

[swvanbuuren]

The chat extension guide doesn't cover chat variables (anymore): In the past it was mentioned that chat variables were in a proposed state, but recently this was removed altogether.

I found out the hard way how to get working chat variables, by browsing VS Code's sources: when adding a tool in package.json, one needs to add the following key/values (in addition to the documented ones):

• "canBeReferencedInPrompt": true
• "userDescription": "Description of chat variable" (optional, shown in menu afaik)
• "icon": "$(note)" (optional, shown next to description in menu)

Only if the first key/value is added, the chat variable is usable in the chat (highlighted in blue, tab-completion works) and becomes available as tool. I suspect there are also other key/values that are also worth mentioning in relation to chat variables.

Please add guidance to the chat extension guide and tools guide on how to create new chat variables for GitHub Copilot chat.