feat(leanls): add support for lakefile.toml-only packages (#3238)

This is partially a bugfix, as such packages would appear to function
correctly because we fell back to `lean --server` -- but that is a lie,
as it means `lake` doesn't control the LSP environment (and specifically
it means that shared objects aren't correctly found for a package which
prebuilds them with prebuildModules).

While we're here, remove said ancient Lean 4 logic for versions no one
has used for years, simplifying the startup (and saving a subprocess).
It'd have been better to fail loudly here rather than falling back to
confusing errors.

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