Improved file filtering for the Static Analysis feature (#4570)

* Improved file filtering for the Static Analysis feature

* Better handling of legacy "check_patterns" option

* Rename "check_src_filter" to plural form to better represent functionality

* Move to plural forms of filter variables

platformio/project/options.py

@@ -649,7 +649,8 @@ ProjectOptions = OrderedDict(
             ),
             ConfigEnvOption(
                 group="check",
-                name="check_patterns",
+                name="check_src_filters",
+                oldnames=["check_patterns"],
                 description=(
                     "Configure a list of target files or directories for checking "
                     "(Unix shell-style wildcards)"