[prompts]: add support for 'mode' header metadata

[legomushroom]

Adds support for the mode header metadata that defines what chat mode to use when a prompt in run in the chat widget. Currently:

• mode can be one of 3 string values: 'ask', 'edit', or 'agent'
• if tools metadata is defined then mode is assumed to be agent and any other configured value is ignored and is highlighted with appropriate warning marker in the editor
• likewise, if a prompt file references for other prompt files, the top level prompt file defines overall chat mode value for the entire prompt tree; this has couple of consequences:
  • if a nested prompt file defines a different mode, it gets ignored
  • if a top-level prompt file does not use agent mode, while some nested prompt defines tools, the tools metadata gets ignored
• furthermore, if multiple prompts are attached at the same time, the overall chat mode and tools metadata is computed across all of them; for instance, if there are multiple prompt files attached and each of them define it's own mode, we compute the safest possible mode that will satisfy all of them, where the Ask mode is the safest while Agent is the least safest
• as for the overall tools, those are computed as a union of all tools that the prompt files define

Part of https://github.com/microsoft/vscode-copilot/issues/15596, https://github.com/microsoft/vscode-copilot/issues/15420 and https://github.com/microsoft/vscode-copilot/issues/12203