Add support for error syntax

[richardhenry]

The error syntax is not currently supported. For example:

contract Foo {
    error Bar();
}

The error will not be highlighted (screenshot):

[zarifpour]

Once the above is merged, should be good to go 👍

[zarifpour]

Thanks for opening this issue! Zed recently added support for language servers, so once this is merged, there will also be better support for Solidity development. Cheers 🍻