Detect declarations deprecated with a doc comment

[Vexu]

I already get a compile error when using declarations deprecated with @compileError, seeing the ones only deprecated with doc comments is much more useful.

On a related note, why aren't the deprecated identifiers rendered with a strikethrough?

/// Deprecated
const foo = 123;
const bar = @compileError("foo");
comptime {
    foo;
    bar;
}

vs

/**
 * @deprecated test
 */
const foo = 1;
foo