Add duckdb build/concepts and use SQLGlot to convert BigQuery SQL into other dialects

[alistairewj]

A few things in this PR, mainly to get the concepts to work with duckdb. This builds off #1529 / @SphtKr. However #1529 has the conversion for MIMIC-III, whereas here I've converted MIMIC-IV. It should be straightforward to adapt for MIMIC-III, but I can't explain the parsing errors I'm getting when running it over that folder, so I've left it for a future PR.

• Added build scripts for mimic-iii in duckdb
• Added build scripts for mimic-iv in duckdb
• Added a mimic_utils package that converts files/folders between SQL dialects.
• Use this package to re-convert the postgresql scripts, and remove the old bash script system for MIMIC-IV
• Moved the mapping folder from mimic-iv/concepts/mapping to mimic-iv/mapping

I also overhauled the READMEs a bit for clarity.

[tompollard]

Thanks, looks good to me, with the caveat that I haven't run the code! (and that breaking this into smaller PRs would be nicer). I added a couple of notes about typos but all very minor, so feel free to go ahead and merge. The sqlglot package looks very cool.

How much manual tweaking of the outputs is needed?

Ideally I think we would maintain a single source format (psql/bigquery) and then create other dialects in a GitHub workflow. My preference would be to have dialects on separate branches, rather than trying to maintain them all on main.

[alistairewj]

Thanks for the review! Fixed the typos; regarding some unaddressed points:

• Consistent formatting - sqlfluff does this now via the action, but it only lints newly changed files.
• Renaming SQL to remove db name - for mimic-iv I did this, for mimic-iii I left all the scripts with their dialect prefixes (postgres_ ..., duckdb_ ...).
• We don't need to manually tweak any outputs for MIMIC-IV (though this is because there are Python patches written to parse some unsupported BigQuery functions). For MIMIC-III, I didn't put in the effort to make it all work, but it mostly works I think.