Add LSP struct literal snippet completion after Type.

[itsfuad]

Goal

Deferred follow-up to #133. Not implementing now.

After Type., offer a struct literal completion for .{required fields} with editable tab stops. Applied edit must produce Type.{...} without duplicating the already typed dot.

Requirements

• Preserve existing member completions alongside the struct literal candidate.
• Honor client snippet support; do not send snippet placeholders to clients without support.
• Emit required fields in declaration order and follow existing field default/required-field rules; do not invent new default semantics.
• Add completion and applied-edit tests covering tab stops, existing member candidates, clients with/without snippet support, declaration order, and defaulted fields.

Separate from auto-import operation completion (#74). Syntax migration and grammar publication/extension revision pinning remain in #133.