refactor symbol handling and update tests for document symbols

[DaliVana]

Addresses the bulk of #3232 (scope notes below).

What changed

Symbol kinds are now inferred from initializer expressions. Previously every
const/var declaration was reported as Constant/Variable based solely on the mut
token, so enums, structs, and imports were indistinguishable in the outline:

declaration	before	after
const E = enum {...};	Constant	Enum
const S = struct {...};	Constant	Struct
const U = union(enum) {...};	Constant	Struct¹
const O = opaque {};	Constant	Class¹
const Error = error{...};, A || B	Constant	Enum
const std = @import("std");	Constant	Module
test "foo" {}	Method	Function²

¹ LSP has no Union/Opaque kind; rust-analyzer maps union → Struct the same way.
² LSP has no Test kind; rust-analyzer and gopls also report tests as plain functions.
Also applied to workspace/symbol.

Methods are distinguished from functions. A function whose first parameter type —
after stripping single-item/C pointers — is @This(), the conventional Self, or the
enclosing container's binding name is reported as Method. Slices and many-item
pointers are rejected since they can never be a method receiver ([*c] is accepted
because *T coerces to it). The binding-name rule is scope-accurate: a function in a
nested container taking the outer type is correctly reported as a plain function.

The _ marker of non-exhaustive enums is no longer listed as an EnumMember
(a member explicitly spelled @"_" still is).

ZON files now have a document outline. Previously textDocument/documentSymbol
returned null for .zon files. The value tree is now walked with kinds mirroring the
JSON language service: struct inits → Module, array inits → Array, plus
String/Number/Boolean/Null/EnumMember for scalars. Struct-init fields are named by
their .field name; array elements by index (0, 1, …). Trees with parse errors
return an empty outline (ZON Asts are unusable when the root expr has errors, same
workaround as hover/goto).

Design constraints

Known heuristic limitations (documented in code): self: anytype receivers are not
detected, and top-level functions of a file-as-struct (const Self = @This() sibling
binding) are reported as Function — most clients render Function and Method with the
same icon, so this is invisible in practice.

Not included, possible follow-ups: kind parity for workspace/symbol (TrigramStore
still records only Constant/Variable/Function/Field), and position-asserting tests for
the ZON range synthesis.

Testing

• updated existing document_symbol/workspace_symbols expectations
• new tests: kind inference (imports, error sets, opaque, sized enums, tagged unions),
  method detection (pointer sizes, @This(), Self, nested and function-local
  containers), non-exhaustive enums, and four ZON outlines (build.zig.zon-style
  fixture, scalars, root array, parse errors)