Emit Warning when not using shorthand syntax

[Xavientois]

When this is better supported for macros, we should create a clippy warning to suggest that users make use of the shorthand syntax where possible.

From this comment:

I feel like it should emit a warning much like clippy::redundant_field_names in the future.

This cannot be done until #1970 is closed and this sort of warning is better supported for macros.

[mc1098]

Once #1748 lands then we could add a lint for this? or use the emit_warning! macro that comes with the proc-macro-error dependency in that PR to emit the warning when we are parsing and find an ident that matches the property name.