Manage namespaces / prefix validation with a settings

See eclipse-lemminx/lemminx#960

Signed-off-by: azerr <azerr@redhat.com>

Author: angelozerr

README.md

@@ -84,6 +84,7 @@ The following settings are supported:
 * [`xml.codeLens.enabled`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/CodeLens.md): Enable/disable XML CodeLens. Default is `false`.
 * [`xml.preferences.showSchemaDocumentationType`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Preferences.md#documentation-type): Specifies the source of the XML schema documentation displayed on hover and completion. Default is `all`.
 * [`xml.validation.enabled`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md): Enable/disable all validation. Default is `true`.
+ * [`xml.validation.namespaces.enabled`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md#xmlvalidationsnamespacesenabled): Enable/disable namespaces validation. Default is `always`. Ignored if [`xml.validation.enabled`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md) is set to `false`.
  * [`xml.validation.schema.enabled`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md#xmlvalidationschemaenabled): Enable/disable schema based validation. Default is `always`. Ignored if [`xml.validation.enabled`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md) is set to `false`.
  * [`xml.validation.disallowDocTypeDecl`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md#disallow-doc-type-declarations): Enable/disable if a fatal error is thrown if the incoming document contains a DOCTYPE declaration. Default is `false`.
 * [`xml.validation.resolveExternalEntities`](https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md#resolve-external-entities): Enable/disable resolve of external entities. Default is `false`.

docs/Validation.md

@@ -420,6 +420,36 @@ Demonstration of the different behaviour:
 
 ## xml.validation.schema.enabled
 
+The `xml.validation.namespaces.enabled` gives the capability to enable / disable the namespaces (prefix) validation. It can be configured with 3 values:
+
+ * `always`: enable schema based validation.
+ * `never`: disable schema based validation.
+ * `onNamespaceEncountered`: enable namespaces validation only when the root element declares an xmlns namespace.
+
+To understand the `onNamespaceEncountered` settings value, lets go through an example:
+
+Create the XML `foo.xml` file:
+
+```xml
+<foo>
+	<prefix:bar />
+</foo>
+```
+
+In `always` you will have error, in `onNamespaceEncountered` you will have none error.
+
+Declares a namespaces in the foo root element:
+
+```xml
+<foo xmlns="http:foo" >
+	<prefix:bar />
+</foo>
+```
+
+In `onNamespaceEncountered` you will see a prefix error.
+
+## xml.validation.schema.enabled
+
 The `xml.validation.schema.enabled` gives the capability to enable / disable the validation based on XSD. It can be configured with 3 values:
 
  * `always`: enable schema based validation.

package.json

@@ -307,6 +307,22 @@
           "markdownDescription": "Enable/disable all validation. Default is `true`.",
           "scope": "window"
         },
+        "xml.validation.namespaces.enabled": {
+          "type": "string",
+          "default": "always",
+          "enum": [
+            "always",
+            "never",
+            "onNamespaceEncountered"
+          ],
+          "markdownEnumDescriptions": [
+            "Enable namespaces validation.",
+            "Disable namespaces validation.",
+            "Enable namespaces validation only when the root element declares an xmlns namespace."
+          ],
+          "markdownDescription": "Enable/disable namespaces validation. Default is `always`. Ignored if `#xml.validation.enabled#` is set to `false`. See [here](command:xml.open.docs?%5B%7B%22page%22%3A%22Validation%22%2C%22section%22%3A%22xmlnamespaceschemaenabled%22%7D%5D) for more information.",
+          "scope": "window"
+        },
         "xml.validation.schema.enabled": {
           "type": "string",
           "default": "always",