Rework env var for Project Context

We had already QUARTO_PROJECT_DIR
https://github.com/quarto-dev/quarto-cli/blob/1573bc693c174b89ac93d9cde4f051c215e630b1/src/command/render/render-shared.ts#L59-L62

https://github.com/quarto-dev/quarto-cli/blob/1573bc693c174b89ac93d9cde4f051c215e630b1/src/project/serve/serve.ts#L213-L214

Use for Lua I believe
https://github.com/quarto-dev/quarto-cli/blob/1573bc693c174b89ac93d9cde4f051c215e630b1/src/resources/pandoc/datadir/init.lua#L1748-L1750

Is QUARTO_PROJECT_ROOT different? it seems to we could use the same, isn't it ?

I think the difference is that QUARTO_PROJECT_ROOT would be set even in non-project mode.

We now always operate in "project mode", so we should consider merging these two environment variables. It would break backwards compatibility, though, so let's not do it right away.

We should also consider adding a QUARTO_IS_SINGLE_FILE_PROJECT boolean env.

Originally posted by @cscheid in https://github.com/quarto-dev/quarto-cli/issues/8525#issuecomment-2045337630