caddyfile: Implement variadics for import args placeholders (#5249)

* implement variadic placeholders
imported snippets reflect actual lines in file

* add import directive line number for imported snippets
add tests for parsing

* add realfile field to help debug import cycle detection.

* use file field to reflect import chain

* Switch syntax, deprecate old syntax, refactoring

- Moved the import args handling to a separate file
- Using {args[0:1]} syntax now
- Deprecate {args.*} syntax
- Use a replacer map for better control over the parsing
- Add plenty of warnings when invalid placeholders are detected
- Renaming variables, cleanup comments for readability
- More tests to cover edgecases I could think of
- Minor cleanup to snippet tracking in tokens, drop a redundant boolean field in tokens

---------

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

caddyconfig/caddyfile/lexer.go

@@ -36,14 +36,31 @@ type (
 	// Token represents a single parsable unit.
 	Token struct {
 		File        string
+		origFile    string
 		Line        int
 		Text        string
 		wasQuoted   rune // enclosing quote character, if any
-		inSnippet   bool
 		snippetName string
 	}
 )
 
+// originalFile gets original filename before import modification.
+func (t Token) originalFile() string {
+	if t.origFile != "" {
+		return t.origFile
+	}
+	return t.File
+}
+
+// updateFile updates the token's source filename for error display
+// and remembers the original filename. Used during "import" processing.
+func (t *Token) updateFile(file string) {
+	if t.origFile == "" {
+		t.origFile = t.File
+	}
+	t.File = file
+}
+
 // load prepares the lexer to scan an input for tokens.
 // It discards any leading byte order mark.
 func (l *lexer) load(input io.Reader) error {