feat: conditionallyExecuteCommand API proposal

[joe-p]

This PR is based on @jrieken comment for allowing access to context through the API:

You should not be reading and evaluating context keys. Instead you should be more like an "event source" and we should own the tuple [event, when-clause, command] - just like we do it for keybindings. Something like this

1. extension registers event source
2. vscode allows to bind events to commands using when-clauses
3. extension fires event -> vscode evaluates the binding (using the focused/active element) and invokes the command

To implement this, I've added a new function to the commands API that allows conditional execution of commands based on the context of the active element. This could be a separate functionality from commands, but I figured it was better to add on to the existing commands implementation rather than creating an entirely new event registration system.

Tests implemented in:

• extensions/vscode-api-tests/src/singlefolder-tests/commands.test.ts
• src/vs/workbench/api/test/browser/mainThreadCommands.test.ts

Closes #249741

Addresses common requests in #10471

[joe-p]

The linter is throwing an error here, but it was not obvious how I should fix it

[joe-p]

@microsoft-github-policy-service agree