feat: add 'simplify variable initialization' code action

[Metbcy]

Adds a refactoring code action that simplifies variable declarations by hoisting type information into the type annotation:

// Before
const foo = @as(u32, 42);
const bar = Point{ .x = 1 };

// After
const foo: u32 = 42;
const bar: Point = .{ .x = 1 };

Handles both @as(T, value) builtin calls and typed struct initializers (T{ ... }). Skips declarations that already have explicit type annotations.

Includes tests for both patterns plus an empty struct init case.

Closes #2098