Add weak_handle! convenience macro

[SludgePhD]

Objective

• A common bevy pattern is to pre-allocate a weak Handle with a static, random ID and fill it during Plugin::build via load_internal_asset!
• This requires generating a random 128-bit number that is interpreted as a UUID. This is much less convenient than generating a UUID directly, and also, strictly speaking, error prone, since it often results in an invalid UUIDv4 – they have to follow the pattern xxxxxxxx-xxxx-4xxx-xxxx-xxxxxxxxxxxx, where x is a random nibble (in practice this doesn't matter, since the UUID is just interpreted as a bag of bytes).

Solution

• Add a weak_handle! macro that internally calls uuid::uuid! to parse a UUID from a string literal.
• Now any random UUID generation tool can be used to generate an asset ID, such as uuidgen or entering "uuid" in DuckDuckGo.

Previously:

const SHADER: Handle<Shader> = Handle::weak_from_u128(314685653797097581405914117016993910609);

After this PR:

const SHADER: Handle<Shader> = weak_handle!("1347c9b7-c46a-48e7-b7b8-023a354b7cac");

Note that I did not yet migrate any of the existing uses. I can do that if desired, but want to have some feedback first to avoid wasted effort.

Testing

Tested via the included doctest.