Terminal suggest: Support configuring fig specs

[Tyriar]

Since we will never be able to ship all the completions that users want and may ship ones that the user does not want (#239019) it would be great to allow users to add their own fig specs to say <user_data_dir>/terminalcompletions or something.

[Tyriar]

We can also use this as a caching mechanism for something like #240724. Also consider workspace-level fig specs so projects can have their own completions.