Make "Go to Symbol in Workspace" more discoverable

We've had a lot of people ask about an equivalent to RStudio's Cmd + .. There is no direct equivalent in VS Code; in VS Code you can get a fuzzy finder that looks for files (Cmd + P) or workspace symbols (Cmd + T) but not both.

It might be okay for RStudio users to need to learn that they need to specify whether they are looking for a file or a function, except that it's very difficult to find the Cmd + T binding! If you click the Search bar it isn't even mentioned:

It's buried beneath the Help option; if you click or type ?, it's in the extended list of commands:

We should make it easier to find the keybinding; at the very least we could add it to the front-page list of keybindings.

(It also feels like it'd be useful to build a finder that merges files and workspace symbols, as there are unlikely to be an overwhelming number of them in most data science projects, but that'd be significantly more expensive so we'd need more evidence it'd be helpful)

See also #669 for a discussion on Cmd + .

Start by reviewing the Search bar and Help keybinding list shown in the issue, then locate the front-page keybinding list in the Positron interface. Make the Cmd+T workspace-symbol command easier to discover, and verify that the keybinding appears where users look for search commands.