fix: enable Julia cell execution in Positron

[DevenVGokhale]

Summary

• Adds case "julia" to the Positron hooks executor (hooks.ts) so Julia chunks in .qmd files route through Positron's runtime API (workbench.action.positronConsole.executeCode), the same way Python and R already do
• Adds ntluong95.positron-julia as an accepted Julia extension ID in executors.ts, since julialang.language-julia is incompatible with Positron due to a VS Code engine version mismatch

Root cause

In hooks.ts, the cellExecutorForLanguage switch only handled "python", "csharp", and "r". Julia had no case, so the function returned undefined, causing:

Cannot read properties of undefined (reading 'executeInREPL')

The fix is a one-line addition of case "julia": to fall through into the same Positron runtime executor path used by R and Python.

Additionally, julialang.language-julia cannot be installed in Positron (VS Code engine version mismatch), so the community extension ntluong95.positron-julia is the only available Julia extension for Positron users. Adding it to requiredExtension allows the extension check to pass.

Test plan

• Open a .qmd file with engine: julia in Positron with ntluong95.positron-julia installed
• Confirm executing a {julia} code chunk sends code to the Positron Julia console
• Confirm variables appear in the Variables pane and plots in the Plots pane
• Confirm Python and R chunk execution is unaffected

Closes #988

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