fix: proper Typescript typings for Seq.concat()

[alexvictoor]

This PR fixes an issue occurring when a sequence is typed as a regular Seq (not an IndexedSeq, keyedSeq, ...).
Below a code fragment to reproduce the issue:

let seq = Seq();
seq = seq.concat(1): // TS error here because seq.concat() return type is Collection<unknown, unknown> instead of Seq<unknown, unknown>