Allow leading underscore in sketch filenames

The Arduino Sketch Specification defines the allowed format of sketch folder names and sketch code filenames. Arduino
Lint's rule SS002 checks compliance with the specification.

The Arduino Sketch Specification has been changed to allow a leading underscore in sketch folder names and sketch code
filenames so the Arduino Lint rule code must be updated accordingly.

Author: per1234

internal/rule/rulefunction/rulefunction.go

@@ -105,7 +105,7 @@ func brokenOutputList(list []string) string {
 
 // validProjectPathBaseName checks whether the provided library folder or sketch filename contains prohibited characters.
 func validProjectPathBaseName(name string) bool {
-	baseNameRegexp := regexp.MustCompile("^[a-zA-Z0-9][a-zA-Z0-9_.-]*$")
+	baseNameRegexp := regexp.MustCompile("^[a-zA-Z0-9_][a-zA-Z0-9_.-]*$")
 	return baseNameRegexp.MatchString(name)
 }