Add Positron context key

[sharon-wang]

Background

We currently set POSITRON=1 in our Terminals, but there are areas where we may want to interface with a context key instead of an environment variable to determine that we're running in Positron.

For example, when clauses could make use of an isPositronContext context key to enable contributions:

https://github.com/posit-dev/positron/blob/d753d0dce92a116a0c6fe6e70cf7822b8e4243a2/extensions/positron-python/package.json#L89-L96

Similarly, enablement clauses could leverage the context key.

https://github.com/posit-dev/positron/blob/d753d0dce92a116a0c6fe6e70cf7822b8e4243a2/extensions/positron-python/package.json#L302-L308

Extensions may want to contribute commands or UI based on if the extension is run in Positron or not.

Proposal

• Create a new context key, such as IsPositron or IsPositronContext
  • Maybe here? https://github.com/posit-dev/positron/blob/main/src/vs/platform/contextkey/common/contextkeys.ts
• These files are also related:
  • https://github.com/posit-dev/positron/blob/main/src/vs/workbench/browser/contextkeys.ts
  • https://github.com/posit-dev/positron/blob/main/src/vs/workbench/common/contextkeys.ts

Notes

• where we set POSITRON=1 https://github.com/posit-dev/positron/blob/d753d0dce92a116a0c6fe6e70cf7822b8e4243a2/src/vs/platform/terminal/node/terminalEnvironment.ts#L144-L153
  • Search for 'POSITRON': '1' for other instances where we set the environment variable
  • Extensions such as Quarto and Workbench use this environment variable to check if the extensions are running in Positron to apply customized behaviour.
• https://code.visualstudio.com/api/references/when-clause-contexts

[juliasilge]

Related to https://github.com/posit-dev/positron/issues/526, and maybe a better way to address that need than something in the Positron API