Resolve keybinding conflict for code cells in .R files (#8888)

Addresses #8637


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#### New Features

- N/A

#### Bug Fixes

- Resolve keyboard shortcut conflict for code cells in `.R` files
(#8637)


### QA Notes

If you have R code like this in a regular `.R` file, with your cursor in
one of the code cells:

```r
# %%
plot(1:10)

# %%
sample(1:100, 10)

# %%
plot(pressure)
```

the keyboard shortcut <kbd>Cmd/Ctrl+Shift+Enter</kbd> should run the
current code cell, and should _not_ `source()` the current file. You can
still `source()` the current file from the editor action bar, the
context menu, the command palette, etc.

Author: juliasilge

extensions/positron-r/package.json

@@ -485,7 +485,7 @@
         "command": "r.sourceCurrentFile",
         "key": "ctrl+shift+enter",
         "mac": "cmd+shift+enter",
-        "when": "editorLangId == r && !isRPackage"
+        "when": "editorLangId == r && !isRPackage && !positron.hasCodeCells"
       },
       {
         "command": "r.insertSection",