Correct the schema for the `output` attribute in Bake files

[rcjsuen]

docker-language-server/internal/bake/hcl/parser/schema.go

Lines 199 to 202 in bd9f041

	"output": {
	IsOptional: true,
	Constraint: schema.List{Elem: schema.AnyExpression{OfType: cty.String}},
	},

output is not just a list of strings. It seems like it can also be an object.

target "_common" {
  args = {
    GO_VERSION = "1.23"
    BUILDKIT_CONTEXT_KEEP_GIT_DIR = 1
  }
}

target "lint" {
  inherits = ["_common"]
  dockerfile = "./dockerfiles/lint.Dockerfile"
  output = [{ type = "cacheonly" }]
}

target "docs" {
  inherits = ["_common"]
  dockerfile = "./dockerfiles/docs.Dockerfile"
  output = ["./docs/reference"]
}

{
  "context": ".",
  "dockerfile": "./dockerfiles/docs.Dockerfile",
  "args": {
    "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
    "GO_VERSION": "1.23"
  },
  "output": [
    {
      "dest": "./docs/reference",
      "type": "local"
    }
  ]
}

{
  "context": ".",
  "dockerfile": "./dockerfiles/lint.Dockerfile",
  "args": {
    "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
    "GO_VERSION": "1.23"
  },
  "output": [
    {
      "type": "cacheonly"
    }
  ]
}