This removes the "threaded" usage of IdeCodeIndexer and moves to
our new async/finish API.

It also creates a new CXIndex for each file, so we can more
reliably release memory after each parse. It probably means we
have to index some data more, but since this is a background
operation, I'd prefer to keep the memory usage lower at any
given time.

We don't want "threaded plugin API". Instead, we want objects that live in
the main thread, and have async/finish API (for which plugins can choose to
use threads if they like).

There is still plenty more to do, but this is a good check-in point
before fixing up things more.

This isn't needed, so just remove it.

This adds more checks to ensure that plugins aren't using the engine
from threads. This cause some issues in the code-index that need to be
fixed.

Recent language servers added a URI variant for this, deprecating the
rootPath path.

This tries a bit harder to ensure we have valid state going forward.

Ensure we don't have things accessing this from other threads, since it is
not safe to do so.

This is a a better type instance check since we use the actual value from
the hash table.

This allows the user to specify CMAKE=/path/to/cmake in case they need to
use an alternate cmake than the default cmake in $PATH.

This uses a fairly common convention to use a "run" target in the Makefile
to run the project. We still require that there is an install target
because the run manager requires that the project is installed before it
can be run.

We want to allow PATH to be overridden so that new PATH parts
can be appended. Such is used by SDK extensions.

If the builddir attribute is set, we need to propagate it to the
build configuration so the build system can potentially override
the build directory that will be used.

We don't need a local cancellable.

Special case, would be nice if this was abstracted but for now
we can just use the _build as convention.

This is useful in situations where we need to getenv, append,
setenv.

This is useful to clear existing values.

If the build configuration requires that the build is in tree,
then use _build inside the source directory.

This can be needed sometimes, such as some meson scripts in the
wild that integrate with Cargo.