### Bug description

`quarto render index.qmd --to pdf,html` fails with `ERROR: Unknown format pdf,html` - but [it seems that should work](https://stackoverflow.com/a/73256116).

### Steps to reproduce

I suppose you could reproduce with any document, but try:

```
git clone https://github.com/matthew-brett/test-quarto
cd test-quarto
quarto render index.qmd --to pdf,html
```

### Expected behavior

Quarto should render the given document or directory to both PDF and HTML formats.

### Actual behavior

```
ERROR: Unknown format pdf,html

Stack trace:
    at resolveFormats (file:///Users/mb312/dev_trees/quarto-cli/src/command/render/render-contexts.ts:591:13)
    at eventLoopTick (ext:core/01_core.js:181:11)
    at async renderContexts (file:///Users/mb312/dev_trees/quarto-cli/src/command/render/render-contexts.ts:231:19)
    at async Object.bookIncrementalRenderAll [as incrementalRenderAll] (file:///Users/mb312/dev_trees/quarto-cli/src/project/types/book/book-render.ts:646:22)
    at async renderProject (file:///Users/mb312/dev_trees/quarto-cli/src/command/render/project.ts:120:5)
    at async Command.fn (file:///Users/mb312/dev_trees/quarto-cli/src/command/render/cmd.ts:212:26)
    at async Command.execute (file:///Users/mb312/dev_trees/quarto-cli/src/vendor/deno.land/x/cliffy@v0.25.4/command/command.ts:1790:7)
    at async quarto (file:///Users/mb312/dev_trees/quarto-cli/src/quarto.ts:135:3)
    at async file:///Users/mb312/dev_trees/quarto-cli/src/quarto.ts:167:5
```

### Your environment

Macos Ventura 13.4
Latest `quarto-cli` commit 1f3581304

### Quarto check output

```
$ quarto check 

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.2: OK
      Dart Sass version 1.55.0: OK
      Deno version 1.33.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      Path: /Users/mb312/dev_trees/quarto-cli/package/dist/bin

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

[✓] Checking Python 3 installation....OK
      Version: 3.10.11
      Path: /Users/mb312/.virtualenvs/resampling-with/bin/python3
      Jupyter: 5.3.0
      Kernels: python3, ir

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

[✓] Checking R installation...........OK
      Version: 4.3.0
      Path: /opt/homebrew/Cellar/r/4.3.0_1/lib/R
      LibPaths:
        - /Users/mb312/Library/R/arm64/4.3/library
        - /opt/homebrew/lib/R/4.3/site-library
        - /opt/homebrew/Cellar/r/4.3.0_1/lib/R/library
      knitr: 1.42
      rmarkdown: 2.21

[✓] Checking Knitr engine render......OK
```
cscheid
2023-06-16T11:23:20Z
matthew-brett
bug triaged-to projects
open
CLI does not recognize multiple output formats in Quarto projects
https://github.com/quarto-dev/quarto-cli/issues/5947