create project can now open in Positron

[cderv]

This PR teaches Quarto about Positron as an editor for quarto create project

• Either to select using quarto create project --open positron
• Either to detect when command is ran into Positron directly

Help page shows the new support

❯ quarto create --help

Usage:   quarto create [type] [commands...]
Version: 99.9.9

Description:

  Create a Quarto project or extension

Options:

  -h, --help              - Show this help.
  --open        [editor]  - Open new artifact in this editor (positron, vscode, rstudio)
  --no-open               - Do not open in an editor

(...)

I checked the Windows paths for default installation of both installers.

This solves #10268

[cderv]

I added isPositronTerminal() and isPositron() to match with other editor, as the env var is POSITRON and not terminal specific it seems. TERM_PROGRAM is vscode like for VSCODE so not helpful.

I asked confirmation for the env var to check to Positron team.

[cderv]

Working on Windows (tested by me) and Mac (tested by @gordonwoodhull)