style: use decl literals

This commit changes a LOT of areas of the code to use decl literals
instead of redundantly referring to the type.

These changes were mostly driven by some regex searches and then manual
adjustment on a case-by-case basis.

I almost certainly missed quite a few places where decl literals could
be used, but this is a good first step in converting things, and other
instances can be addressed when they're discovered.

I tested GLFW+Metal and building the framework on macOS and tested a GTK
build on Linux, so I'm 99% sure I didn't introduce any syntax errors or
other problems with this. (fingers crossed)

src/crash/sentry_envelope.zig

@@ -331,7 +331,7 @@ pub const Item = union(enum) {
 
         // Decode the item.
         self.* = switch (encoded.type) {
-            .attachment => .{ .attachment = try Attachment.decode(
+            .attachment => .{ .attachment = try .decode(
                 alloc,
                 encoded,
             ) },