Add missing dependencies entry for which, which was causing the w…

…rong version of `which` to be used, which caused R not to load on some configurations of Windows (#4311)

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This PR addresses #4280.

This issue was tracked down to the fact that the `positron-r` extension
did not have an explicit `dependencies` entry for the `which` package,
but it did have a `devDependencies` entry for `@types/which`
(`"@types/which": "^3.0.0",`).

As a result of this, the `positron-r/node_modules` folder contained
`which` 2.0.2 (I believe) from:

```
mocha@^9.2.1:
  version "9.2.2"
  resolved "https://registry.npmjs.org/mocha/-/mocha-9.2.2.tgz"
  integrity sha512-L6XC3EdwT6YrIk0yXpavvLkn8h+EU+Y5UcCHKECyMbdUIxyMuZj4bX4U9e1nvnvUUvQVsV2VHQr5zLdcUkhW/g==
  dependencies:
    "@ungap/promise-all-settled" "1.1.2"
    ansi-colors "4.1.1"
    browser-stdout "1.3.1"
    chokidar "3.5.3"
    debug "4.3.3"
    diff "5.0.0"
    escape-string-regexp "4.0.0"
    find-up "5.0.0"
    glob "7.2.0"
    growl "1.10.5"
    he "1.2.0"
    js-yaml "4.1.0"
    log-symbols "4.1.0"
    minimatch "4.2.1"
    ms "2.1.3"
    nanoid "3.3.1"
    serialize-javascript "6.0.0"
    strip-json-comments "3.1.1"
    supports-color "8.1.1"
    which "2.0.2"
    workerpool "6.2.0"
    yargs "16.2.0"
    yargs-parser "20.2.4"
    yargs-unparser "2.0.0"
```

This version of `which` does not support the `nothrow` option for `async
which`, which we rely on in this code fragment from
`extensions/positron-r/src/provider.ts`:

```
	const whichR = await which('R', { nothrow: true }) as string;
```

Because of this, on some configurations of Windows, an error was being
thrown which caused R not to load.

For background on this, see:
npm/node-which#80. This was the exact same
problem.

### QA Notes

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extensions/positron-r/package.json

@@ -598,6 +598,7 @@
     "split2": "^4.2.0",
     "vscode-languageclient": "^9.0.1",
     "web-tree-sitter": "^0.20.8",
+    "which": "^3.0.0",
     "winreg": "^1.2.5"
   },
   "repository": {

extensions/positron-r/yarn.lock

@@ -2315,6 +2315,13 @@ [email protected], which@^2.0.1:
   dependencies:
     isexe "^2.0.0"
 
+which@^3.0.0:
+  version "3.0.1"
+  resolved "https://registry.yarnpkg.com/which/-/which-3.0.1.tgz#89f1cd0c23f629a8105ffe69b8172791c87b4be1"
+  integrity sha512-XA1b62dzQzLfaEOSQFTCOd5KFf/1VSzZo7/7TUjnya6u0vGGKzU96UQBZTAThCb2j4/xjBAyii1OhRLJEivHvg==
+  dependencies:
+    isexe "^2.0.0"
+
 winreg@^1.2.5:
   version "1.2.5"
   resolved "https://registry.yarnpkg.com/winreg/-/winreg-1.2.5.tgz#b650383e89278952494b5d113ba049a5a4fa96d8"