Support stable rust

[chenrui333]

Problem

Currently, starpls uses trait upcasting coercion—a nightly-only feature tracked under rust-lang/rust#65991. As a result, compilation on stable Rust triggers errors like:

error[E0658]: cannot cast `dyn Db` to `dyn starpls_common::Db`, trait upcasting coercion is experimental
   ...
   = note: see issue #65991 for more information

[withered-magic]

Fixed after https://github.com/withered-magic/starpls/pull/390