Refer to the platform "extra_flags" properties in a less confusing manner

There is a convention of configuring Arduino boards platforms to provide a user interface for configuring the build
process through the addition of references and fallback empty definitions of a series of properties that follow the
format of `compiler.<pattern type>.extra_flags`, where "<pattern type>" corresponds to the compilation pattern that
references them:

- `c`
- `cpp`
- `S`
- `ar`
- `c.elf`

Previously, `x` was used as a placeholder for the component of the property name that varies in order to allow a single
rule to refer to the entire set. "<pattern type>" is a little bit more clear than "x".

Author: per1234

Diff for: internal/rule/ruleconfiguration/ruleconfiguration.go

@@ -1696,8 +1696,8 @@ var configurations = []Type{
 		Category:         "configuration files",
 		Subcategory:      "boards.txt",
 		ID:               "PF009",
-		Brief:            "use of compiler.x.extra_flags",
-		MessageTemplate:  "Board ID(s) {{.}} use compiler.x.extra_flags properties. These are intended to be left for use by the user.",
+		Brief:            "use of compiler.<pattern type>.extra_flags",
+		MessageTemplate:  "Board ID(s) {{.}} use compiler.<pattern type>.extra_flags properties. These are intended to be left for use by the user.",
 		Description:      "A board definition in the platform's `boards.txt` configuration file is using one of the `compiler.<pattern type>.extra_flags` properties (e.g., `compiler.cpp.extra_flags`). These are intended to be left for use by the user as a standardized interface for customizing the compilation commands. The platform author can define as many arbitrary properties as they like, so there is no need for them to take the user's properties.",
 		Reference:        "",
 		DisableModes:     nil,

Diff for: internal/rule/rulefunction/platform.go

@@ -169,7 +169,7 @@ func BoardsTxtBoardIDBuildCoreLTMinLength() (result ruleresult.Type, output stri
 	return ruleresult.Pass, ""
 }
 
-// BoardsTxtUserExtraFlagsUsage checks if the user's compiler.x.extra_flags properties are used in boards.txt.
+// BoardsTxtUserExtraFlagsUsage checks if the user's compiler.<pattern type>.extra_flags properties are used in boards.txt.
 func BoardsTxtUserExtraFlagsUsage() (result ruleresult.Type, output string) {
 	if projectdata.BoardsTxtLoadError() != nil {
 		return ruleresult.NotRun, "Couldn't load boards.txt"