lib.rs: Update SignatureHelp parameter types
lib.rs: Update SignatureHelp parameter types

According to LSP spec, SignatureHelp::active_parameter and
SignatureHelp::active_signature have type `number` which can have negative
values as well.

Change the type of parameters in the struct from u64 to i64 to allow
de-serializing from servers that set the value of these parameters to -1 to
indicate value that is ignored.
gluon-lang/languageserver-types