Add new formatting setting xml.format.numberOfIndentsForSplitAttributes

Add a new setting which controls the level of indentation of attributes
with respect to their parent element when `xml.format.splitAttributes`
is set to `true`.

Based off of a comment by Cong Wang (@I322871) on the vscode-xml Gitter

Signed-off-by: David Thompson <davthomp@redhat.com>

Author: datho7561

docs/Formatting.md

@@ -148,7 +148,8 @@ If this value is set to 0, then all blank lines will be removed during formattin
 ### xml.format.splitAttributes
 
   Set to `true` to split node attributes onto multiple lines during formatting. Defaults to `false`.
-  Overrides the behaviour of [xml.format.preserveAttributeLineBreaks](#xmlformatpreserveattributelinebreaks)
+  Overrides the behaviour of [xml.format.preserveAttributeLineBreaks](#xmlformatpreserveattributelinebreaks).
+  Please see [xml.format.numberOfIndentsForSplitAttributes](#xmlformatnumberOfIndentsForSplitAttributes) for information on configuring the indentation level of the attributes.
 
   ```xml
   <project a="1" b="2" c="3"></project>
@@ -332,4 +333,66 @@ If it is set to `true`, the above document becomes:
     ```
 
 ***
+### xml.format.numberOfIndentsForSplitAttributes
 
+  Use to configure how many levels to indent the attributes by when `xml.format.splitAttributes` is set to `true`.
+
+  Here are some examples. For these examples, an indentation is two spaces.
+
+  `xml.format.numberOfIndentsForSplitAttributes = 2` (default)
+
+  ```xml
+  <robot attribute1="value1" attribute2="value2" attribute3="value3">
+    <child />
+    <child />
+  </robot>
+  ```
+  becomes
+  ```xml
+  <robot
+      attribute1="value1"
+      attribute2="value2"
+      attribute3="value3">
+    <child />
+    <child />
+  </robot>
+  ```
+
+  `xml.format.numberOfIndentsForSplitAttributes = 1`
+
+  ```xml
+  <robot attribute1="value1" attribute2="value2" attribute3="value3">
+    <child />
+    <child />
+  </robot>
+  ```
+  becomes
+  ```xml
+  <robot
+    attribute1="value1"
+    attribute2="value2"
+    attribute3="value3">
+    <child />
+    <child />
+  </robot>
+  ```
+
+  `xml.format.numberOfIndentsForSplitAttributes = 3`
+
+  ```xml
+  <robot attribute1="value1" attribute2="value2" attribute3="value3">
+    <child />
+    <child />
+  </robot>
+  ```
+  becomes
+  ```xml
+  <robot
+        attribute1="value1"
+        attribute2="value2"
+        attribute3="value3">
+    <child />
+    <child />
+  </robot>
+  ```
+***

package-lock.json

@@ -257,6 +257,11 @@
           "markdownDescription": "Split multiple attributes each onto a new line. Default is `false`. See [here](command:xml.open.docs?%5B%7B%22page%22%3A%22Formatting%22%2C%22section%22%3A%22xmlformatsplitattributes%22%7D%5D) for more information.",
           "scope": "window"
         },
+        "xml.format.numberOfIndentsForSplitAttributes": {
+          "type": "number",
+          "default": 2,
+          "markdownDescription": "How many levels to indent the attributes by when `xml.format.splitAttributes` is `true`. See [here](command:xml.open.docs?%5B%7B%22page%22%3A%22Formatting%22%2C%22section%22%3A%22xmlformatnumberofindentsforsplitattributes%22%7D%5D) for more information"
+        },
         "xml.preferences.quoteStyle": {
           "type": "string",
           "enum": [