Basic implementation for piercing Shadow DOM

- Added test case for looking into the nested shadow dom
- Added basic method for piercing the showdom dom. Here using the cascading locators
  and checking if a parent element is a shadow dom host. If so it replaces the parent
  element with the resulting shadow dom object. Thus the continued find then searches
  the shadow dom root instead of the regular dom. This is a highly simplified method
  for piercing and misses many important scenarios (might be a descendant but on the
  regular dom for example).

atest/acceptance/3-shadow_dom/piercing_shadowdom.robot

@@ -11,3 +11,6 @@ Test Setup        Go To Page "shadow_dom.html"
 Multiple Locators with double arrows as separator should work
     [Setup]    Go To Page "links.html"
     Page Should Contain Element    css:div#div_id >> xpath:a[6] >> id:image1_id
+
+Nested Shadow DOM
+    Page Should Contain Element    id:shadow_host >> id:nested_shadow_host >> id:nested_div

src/SeleniumLibrary/locators/elementfinder.py

@@ -21,6 +21,7 @@
 from selenium.webdriver.remote.webelement import WebElement
 from selenium.webdriver.support.event_firing_webdriver import EventFiringWebElement
 from selenium.webdriver.common.by import By
+from selenium.common.exceptions import NoSuchShadowRootException
 
 from SeleniumLibrary.base import ContextAware
 from SeleniumLibrary.errors import ElementNotFound
@@ -110,13 +111,21 @@ def _split_locator(self, locator: Union[str, list]) -> list:
         return parts
 
     def _find(self, locator, tag=None, first_only=True, required=True, parent=None):
+        parent_is_shadowdom_host = False
         element_type = "Element" if not tag else tag.capitalize()
         if parent and not self._is_webelement(parent):
             raise ValueError(
                 f"Parent must be Selenium WebElement but it was {type(parent)}."
             )
         if self._is_webelement(locator):
             return locator
+        try:
+            if parent:
+                parent_is_shadowdom_host = parent.shadow_root
+        except NoSuchShadowRootException:
+            pass
+        if parent_is_shadowdom_host:  # and ??._shadowdom_piercing
+            parent = parent_is_shadowdom_host
         prefix, criteria = self._parse_locator(locator)
         strategy = self._strategies[prefix]
         tag, constraints = self._get_tag_and_constraints(tag)