Add OpenRPC method for evaluating a when clause

[juliasilge]

Goes along with posit-dev/ark#449

I was thinking about how to address #2697 and have a proposed approach in this PR plus the accompanying PR for ark. I looked at how various extensions (like the git extension, etc) check for whether we are in a git repo in the workspace and all that checking is typically based on context keys. In an extension specifically, you can check those keys with a "when" clause and I realized that approach (i.e. a string) could work well as an OpenRPC method and would be flexible for other context keys we will need to check when we are not in the main thread.

Alternatively, we could add methods to return more specific info (are we in a git repo? etc etc etc) but I like the idea of using "when" clauses for this, like an extension could directly for commands, etc.

QA Notes

Evaluating code like this in R should show the correct results for your situation:

.ps.ui.evaluateWhenClause("isLinux || isWindows")
.ps.ui.evaluateWhenClause("isMac")
.ps.ui.evaluateWhenClause("gitOpenRepositoryCount >= 1")

[jmcphers]

This is very clever and I think it'll be useful in lots more places!

I do think some error handling needs to happen so that an invalid when_clause generates an RPC error that gets propagated to an R error.

[juliasilge]

Notice the behavior here:

positron/src/vs/platform/contextkey/browser/contextKeyService.ts

Line 322 in 8947e98

const result = (rules ? rules.evaluate(context) : true);

If you do something like "logLevel == debug", then ContextKeyExpr.deserialize() gives you something like:

{
  key: "logLevel",
  value: "debug",
  negated: null,
  type: 4,
}

If you do something like "no one should eat mashed potatoes", then ContextKeyExpr.deserialize() gives you undefined and then contextKeyService.contextMatchesRules() evaluates to true. In 6939877 I caught all undefined deserialized context keys and added an error.