feat(lsp): show value on hover for const variables and CellPaths (#14940)

# Description

e.g.
<img width="299" alt="image"
src="https://github.com/user-attachments/assets/3c16835a-6d4d-48ec-b7d6-68d5bdb88ea2"
/>

and `goto def` on cell paths now finds the specific cell (only for
const) instead of doing nothing.

# User-Facing Changes
# Tests + Formatting
# After Submitting

crates/nu-protocol/src/ast/cell_path.rs

@@ -68,6 +68,13 @@ impl PathMember {
             } => *optional = true,
         }
     }
+
+    pub fn span(&self) -> Span {
+        match self {
+            PathMember::String { span, .. } => *span,
+            PathMember::Int { span, .. } => *span,
+        }
+    }
 }
 
 impl PartialEq for PathMember {