add code action that simplifies variable initialization #2098

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