Add length to spans and diagnostics

[LeonMatthes]

Previously, diagnostics only indicated the first character of a warning/error.
While this is often sufficient, in many cases it leads to ambiguity as to what exactly causes the error.

This patch updates the Span type to add a length field and exposes this data to the diagnostic builder in the compiler and lsp.

The patch is only large, as the syntax tests all needed to be updated. This was done via the automatic update mechanism.
The actual code changes are pretty straight-forward.

⚠️ I have not updated the ranges on the existing diagnostics. They mostly seem to make sense, but some suboptimal behavior is possible. Let me know if I should update some of the spans to make them fit better.

Also let me know whether the new syntax of > < for the syntax tests is acceptable.

Compiler Diagnostics

Before:

warning: Expression has no effect!
  --> noops.slint:42:9
   |
42 |         true && false;
   |         ^

After:

warning: Expression has no effect!
  --> noops.slint:44:9
   |
44 |         true && false;
   |         ^^^^^^^^^^^^^

LSP (Screenshots from NVIM)

Before:

After: