Support diagnostics on multiple lines

[fm-117]

Following #1845 and to match LSP specifications, our parser should allow definition of diagnostics spanning over multiple lines.
In our current implementation, only line start is considered. When serializing to JSON (for LSP messages) line start is duplicated and send also as line end for each diagnostic.

The goal of this issue is to add a LineEnd property in Diagnostic class and update all usages accordingly.