`filters` and `shortcodes` do not respect Quarto's convention about leading `/`

filters and shortcodes path resolution does not respect Quarto's convention about leading /.

mkdir issue
cd issue
touch -f _quarto.yml
echo 'function Pandoc(doc)\n  quarto.log.output("Hello, world!")\nend' > filter.lua
mkdir subdir

echo '---\ntitle: "Hello"\nformat: html\nfilters:\n  - at: post-quarto\n    path: /filter.lua\n---\n\n# Hello, world!' > subdir/hello.qmd
quarto render subdir/hello.qmd

echo '---\ntitle: "Hello"\nformat: html\nfilters:\n  - /filter.lua\n---\n\n# Hello, world!' > subdir/hello.qmd
quarto render subdir/hello.qmd

echo '---\ntitle: "Hello"\nformat: html\nshortcodes:\n  - /filter.lua\n---\n\n# Hello, world!' > subdir/hello.qmd
quarto render subdir/hello.qmd

ERROR (/Users/mcanouil/Projects/quarto/quarto-cli/src/resources/filters/./common/wrapped-filter.lua:200) cannot open /filter.lua: No such file or directory
ERROR: Error
    at renderFiles (file:///Users/mcanouil/Projects/quarto/quarto-cli/src/command/render/render-files.ts:351:23)
    at eventLoopTick (ext:core/01_core.js:175:7)
    at async renderProject (file:///Users/mcanouil/Projects/quarto/quarto-cli/src/command/render/project.ts:463:23)
    at async Command.actionHandler (file:///Users/mcanouil/Projects/quarto/quarto-cli/src/command/render/cmd.ts:251:26)
    at async Command.execute (https://deno.land/x/cliffy@v1.0.0-rc.3/command/command.ts:1948:7)
    at async Command.parseCommand (https://deno.land/x/cliffy@v1.0.0-rc.3/command/command.ts:1780:14)
    at async quarto (file:///Users/mcanouil/Projects/quarto/quarto-cli/src/quarto.ts:191:5)
    at async file:///Users/mcanouil/Projects/quarto/quarto-cli/src/quarto.ts:220:5
    at async file:///Users/mcanouil/Projects/quarto/quarto-cli/src/core/main.ts:41:14
    at async mainRunner (file:///Users/mcanouil/Projects/quarto/quarto-cli/src/core/main.ts:43:5)