add code action that simplifies variable initialization

[Techatrix]

Before:

const foo = @as(T, something);
const bar = T{};

After:

const foo: T = something;
const bar: T = .{};

This feature has been suggested by @mlugg a while ago but I never found the time to implement it.

Thanks to #2063 it becomes trivial to optionally enable certain code actions to be run on save.

Related ziglang/zig#5038

[srijan-paul]

Hm. This would be nice to have.
If you haven't taken a look by next weekend, I can take a stab at it.

[Techatrix]

@mlugg Since you know so much about RLS, could you tell me whether the following conversion is also valid?

const foo = T{}; // Before
const foo: T = .{}; // After

[mlugg]

Yep, T{ ...} is identical to @as(T, .{ ... }), so that's a valid conversion.