Support symlinked extension directory in `_extensions/`

Start by reproducing the failure with the linked testproject extension: enter its docs directory and run quarto render with the symlink in _extensions/. Trace the projectType and projectContext paths shown in the stack trace, then verify that the same render succeeds with the symlink while preserving custom project type support.

Bug description

In https://github.com/quarto-dev/quarto-cli/discussions/8579 I have been discussing the best way to structure a repository for a Quarto extension alongside a documentation website that uses that extension. @coatless has had success symlinking the doc website's extension folder (whether for a single extension or several) to the parent folder (eg. so if the documentation website is in /docs, /docs/_extensions/testproject -> /_extensions/testproject).

I'm trying to pursue this option, but I find that if my extension involves a custom project type (see reprex extension), I get an error when I render the documentation website.

Steps to reproduce

Clone the reprex extension, then:

$ cd docs
$ quarto render
ERROR: Unsupported project type testproject

Stack trace:
    at projectType (file:///Users/rensa/Applications/quarto/bin/quarto.js:37943:15)
    at projectContext (file:///Users/rensa/Applications/quarto/bin/quarto.js:72786:30)
    at eventLoopTick (ext:core/01_core.js:183:11)
    at async render (file:///Users/rensa/Applications/quarto/bin/quarto.js:81779:19)
    at async Command.fn (file:///Users/rensa/Applications/quarto/bin/quarto.js:81954:32)
    at async Command.execute (file:///Users/rensa/Applications/quarto/bin/quarto.js:8104:13)
    at async quarto (file:///Users/rensa/Applications/quarto/bin/quarto.js:114999:5)
    at async file:///Users/rensa/Applications/quarto/bin/quarto.js:115017:9

Expected behavior

If I delete the symlink and manually copy the files into the doc website's extension folder, it renders fine:

$ cd _extensions
$ rm testproject
$ cp -r ../../_extensions/testproject .
$ quarto render
[1/2] index.qmd
[2/2] about.qmd

Output created: _site/index.html

Your environment

• IDE: none (using terminal)
• macOS: 14.2.1
• Quarto: have tested on both 1.4.550 and pre-release (at commit 581c7e54d)

Quarto check output

Using the pre-release:

Quarto 99.9.9
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.11: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.41.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      Path: /Users/rensa/code/quarto-cli/package/dist/bin

Check file:///Users/rensa/code/quarto-cli/src/resources/vendor/deno-land/x/puppeteer@9-0-2/mod.ts
[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/rensa/Library/TinyTeX/bin/universal-darwin
      Version: 2021

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.9.7 (Conda)
      Path: /Users/rensa/miniforge3/bin/python
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with conda install jupyter

[✓] Checking R installation...........OK
      Version: 4.2.1
      Path: /Library/Frameworks/R.framework/Versions/4.2-arm64/Resources
      LibPaths:
        - /Users/rensa/Library/R/arm64/4.2/library
        - /Library/Frameworks/R.framework/Versions/4.2-arm64/Resources/library
      knitr: 1.42
      rmarkdown: 2.21

[✓] Checking Knitr engine render......OK