Add an explicit dependency on rustc-std-workspace-core

When built as a proc-macro dependency with the `rustc-dep-of-std`
feature, we add an explicit `std` dependency but not one on `core`.
This means that any macros defined in `core` (such as panic-related macros)
that refer to `::core::` will fail to compile when used in
`literal-escaper`.

Work around this by adding an explicit dependency on
`rustc-std-workspace-core` when the `rustc-dep-of-std` feature is
enabled.

Author: paulmenage

Cargo.lock

@@ -8,6 +8,7 @@ repository = "https://github.com/rust-lang/literal-escaper"
 
 [dependencies]
 std = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-std' }
+core = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-core' }
 
 [features]
-rustc-dep-of-std = ["dep:std"]
+rustc-dep-of-std = ["dep:std", "dep:core"]