Add a setting to specify default repositories

[jmcphers]

This change adds a new setting that allows end users to select the default repository used in R sessions. Prior to the change, we always used cran.rstudio.com if no other repository was set; this remains the default, but now there are several other choices:

For compatibility with shared RStudio configurations, the auto setting also checks for a repos.conf file in well-known XDG locations. You can read more about the expected format of this file here: https://solutions.posit.co/envs-pkgs/rsw_defaults/#repos.conf

Addresses #5509

Note

This change depends on a change from ark (posit-dev/ark#645) and should not be merged until that change is.

QA Notes

• These repositories are defaults and shouldn't override anything you've already set in e.g. .Rprofile or Rprofile.site.
• It is not strictly necessary to restart Positron to apply changes to the new option; you only need to shut down your R session (not restart it) and create a new one. We say "restart Positron" because it's easier for people to understand.
• The XDG locations searched include both Positron and RStudio directories. On macOS, for example, repos.conf can be placed in ~/Library/Preferences/positron/repos.conf.

[jennybc]

Leaving two comments this PR inspired. About to try this out locally.

[jennybc]

Minor observation: we are using a little interface for env vars elsewhere in positron-r (but I don't think this matters that much and I see this code pre-existed, it's just moving in this PR):

positron/extensions/positron-r/src/session.ts

Lines 30 to 32 in 1492e58

	export interface EnvVar {
	[key: string]: string;
	}

positron/extensions/positron-r/src/uri-handler.ts

Line 69 in 1492e58

export async function prepCliEnvVars(session?: RSession): Promise<EnvVar> {

positron/extensions/positron-r/src/session.ts

Line 927 in 1492e58

export async function getEnvVars(envVars: string[], session?: RSession): Promise<EnvVar[]> {

[jmcphers]

Good observation! Indeed I didn't touch this (just moved it to keep the file from becoming too unwieldy) but that'd be an improvement.

[jennybc]

Taking this chance to have a side discussion:

In my .Rprofile, I have

...
repos = c(
    P3M = "https://p3m.dev/cran/latest",
    CRAN = "https://cloud.r-project.org"
)
...

In terms of what we feature, is there a reason to push cran.rstudio.com over cloud.r-project.org? I guess I've implicitly preferred the latter from some super vague reasons around rstudio->posit rebrand and avoiding overt branding.

[jmcphers]

I don't think we necessarily want to push cran.rstudio.com (Posit also sponsors cloud.r-project.org), I just used it to match ark's existing behavior. Might be worth providing a cloud.r-project.org option too, let's see what folks say.

[jennybc]

I played around with this, with an appropriate ark version, and all seems well. I can switch between the different enum values and I can also use a repos.conf file with "auto".