Improve control over Pandoc reader extensions

Bug description

Pandoc markdown syntax extension east_asian_line_breaks is quite helpful for writing markdown with CJK characters wrapped in a long paragraph. However, it seems that Quarto will just ignore this extension. While R Markdown with md_extensions: "+east_asian_line_breaks" works fine.

Steps to reproduce

The test Quarto file:

---
format:
    markdown:
        from: "markdown+east_asian_line_breaks"
---

你
好

a
b

The test R Markdown file:

---
output:
    md_document:
        md_extensions: "+east_asian_line_breaks"
---
你
好

a
b

Expected behavior

你
好

The above should be put in a single line without any space between 你 and 好.

The output from RMarkdown file:

你好

a b

Actual behavior

The output from the Quarto file:

---
toc-title: Table of contents
---

你 好

a b

Your environment

• IDE: Neovim
• OS: MacOS

But I confirmed this behaviour is the same across all platforms, including Windows, Linux and MacOS.

Quarto check output

❯ quarto check
Quarto 1.4.549
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.11: OK
      Dart Sass version 1.69.5: OK
      Deno version 1.37.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.4.549
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/hongyuanjia/Library/TinyTeX/bin/universal-darwin
      Version: 2023

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

[✓] Checking Python 3 installation....OK
      Version: 3.9.6
      Path: /Library/Developer/CommandLineTools/usr/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.3.2
      Path: /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources
      LibPaths:
        - /Users/hongyuanjia/Library/R/arm64/4.3/library
        - /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library
      knitr: 1.45
      rmarkdown: 2.25

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