Add package management RPC functions

[bricestacey]

This PR is to move R code from Positron into Ark. This provides two key features:

1. We can use the LanguageRuntimeSession.callMethod() function to execute this code and return the result.
2. The code is guaranteed to be loaded and we therefore do not need to load it from Positron

Related tickets:
posit-dev/positron#12076
posit-dev/positron#11842

Add new RPC endpoints for the Positron packages pane:

• pkg_install: Install packages via pak or base R
• pkg_list: List installed packages with metadata
• pkg_update_all: Update all outdated packages
• pkg_uninstall: Remove packages and unload namespaces
• pkg_search: Search CRAN for packages by name
• pkg_search_versions: Get available versions for a package

Each function supports both pak and base methods to allow flexibility depending on user preference and environment.

Related Positron PR that consumes this change: posit-dev/positron#12280

[juliasilge]

This is working great!

Before you merge this, can you add some comments to each of these new functions to note that they are for use in the Packages Pane with the RPackageManager? I don't want us to get confused in this file between .ps.rpc.install_packages and .ps.rpc.pkg_install. Or maybe add comments and make the prefix for each of these new functions .ps.rpc.pkg_pane_install() and similar? If you're up for pushing a change to both branches.

[bricestacey]

The strategy changed, so I've removed the package mutation functions.

I also moved everything to its own file to keep it separate.

Positron will now use executeCode() to perform package mutation operations. This executes the code directly in the console. We want to execute a "user-friendly" line of code, not one of these functions as we have no intention of creating yet another API for managing packages. So that is your traditional install.packages() or pak::pkg_install().

For install/uninstall it's really just switching between base R and pak and that is now performed in Positron.

However, for pak's update all, we still need to call into Ark for .ps.rpc.pkg_outdated() and feed the results into the code fed to executeCode()...

[juliasilge]

This is working beautifully! 🤩