Support activeparameter property for signatureinformation

Support activeparameter property for signatureinformation

SignatureInformation.activeParameter is new in version 3.16.0 of the
protocol.  When non-nil, it is used in place of
SignatureHelp.activeParameter.  The latter was deemed insufficient in
languages where multiple signatures for the same function may exist
with arbitrary order of parameters, like Python.

Co-authored-by: João Távora <joaotavora@gmail.com>
* eglot.el (eglot--lsp-interface-alist): Add
SignatureInformation.activeParameter.

* eglot.el (eglot--sig-info): Prioritize
SignatureInformation.activeParameter over
SignatureHelp.activeParameter.

GitHub-reference: close https://github.com/joaotavora/eglot/issues/605