fix: push optional-list and optional-map validation to post-parse (#959)

We currently don't allow users to define fields of type `type[]?` or
`map<type, type>?`. (This is because the jsonish coercer defaults unset
to an empty list/map, rather than an unset.)

This validation currently happens at AST parse time, even though there's
no technical constraint requiring us to: this is a perfectly parse-able
grammar instantiation, and so we should really be doing this validation
post-parse, which allows us to also surface diagnostics about other BAML
syntax errors.

In addition, do a massive cleanup pass over the code:

- remove a number of dead codepaths (baml-cli, baml-fmt,
`.serialize_data()` which had previously been used for the WASM layer)
- flag potentially suspect codepaths (these are now marked with some
variant of `TODO` and/or `SUSPECT`)
- set `MACOSX_DEPLOYMENT_TARGET` to 10.13 for the primary build (this
used to be set in `baml-cli/build.rs` but was dropped in baml-runtime)

Author: sxlijin

engine/.gitignore

@@ -2,6 +2,11 @@
 # will have compiled files and executables
 debug/
 target/
+# NOTE(sam): I use this so that rust-analyzer doesn't have to share build directories with
+# the cargo build CLI, by setting
+#    "rust-analyzer.cargo.targetDir": "./target-rust-analyzer",
+# in VSCode settings.
+target-rust-analyzer/
 
 # NOTE(sam): I use this on my own laptop, so that `cargo build` doesn't have
 # to fight rust-analyzer for the build directory lock.