Flag malformed annotation as an error

[rcjsuen]

Since Bake expects annotations to be of the key=value form, we should create an error for this if we detect it in the language server.

target "t1" {
  annotations = [ "abc" ]
}

$ docker buildx bake --print t1
[+] Building 0.0s (1/1) FINISHED
 => [internal] load local bake definitions                                                                                                       0.0s
 => => reading compose.yaml 60B / 60B                                                                                                            0.0s
 => => reading docker-bake.hcl 42B / 42B                                                                                                         0.0s
ERROR: invalid annotation "abc", expected key=value