lexer: allow extglob wildcards as function names

the lexer assumes an extglob token if any of the wildcards expressions
(such as `@`, and `+`) are succeeded by a left parenthesis but that
proves to be an issue if the wildcard is used as a function name.

example input:
```
$ cat in.sh
@() {
  echo "$@";
}
```

`bash` and `gosh` comparison:
```
$ bash ./in.sh
hello
$ ./gosh in.sh
in.sh:5:1: "}" can only be used to close a block
```

given `in.sh`, gosh reports about a syntax error - this is because
a closing bracket is found while the lexer isn't assuming a function
block

fix the issue by assuming a function if one of the conditions below is
true:
* if the expression is found at the beginning of the statement or if its
  preceded by a "function"
* if `(` is immediately succeeded by a `)` - although this is a valid
  bash syntax, we'll operate on the likelihood that it is a function

fixes mvdan#739

interp/interp_test.go

@@ -3117,6 +3117,16 @@ hello, world
 hello, world
 `,
 	},
+	{
+		// globbing wildcard as function name
+		`@() { echo "$@"; }; @ lala; function +() { echo "$@"; }; + foo`,
+		"lala\nfoo\n",
+	},
+	{
+		// globbing wildcard as function name but with space after the name
+		`+ () { echo "$@"; }; + foo; @ () { echo "$@"; }; @ lala; ? () { echo "$@"; }; ? bar`,
+		"foo\nlala\nbar\n",
+	},
 }
 
 var runTestsWindows = []runTest{

syntax/lexer.go

@@ -290,7 +290,7 @@ skipSpace:
 				p.advanceLitNone(r)
 			}
 		case '?', '*', '+', '@', '!':
-			if p.peekByte('(') {
+			if p.isExprGlob() {
 				switch r {
 				case '?':
 					p.tok = globQuest
@@ -346,6 +346,26 @@ skipSpace:
 	}
 }
 
+// isExprGlob determines whether the expression is an extglob wildcard literal
+func (p *Parser) isExprGlob() bool {
+	leftParen := p.peekByte('(')
+	if !leftParen {
+		return false
+	}
+	// it must not be a globbing if its at the beginning of a statement or
+	// if its preceded by "function"
+	if p.pos.Col() == 1 || p.val == "function" {
+		return false
+	}
+	// its must not be a globbing if left parens is succeeded by a right parens
+	// NOTE: empty pattern list is a valid globbing syntax, eg @()
+	// but we'll operate on the "likelihood" that it is a function
+	if len(p.bs) > p.bsp+p.w {
+		return p.bs[p.bsp+p.w] != ')'
+	}
+	return true
+}
+
 func (p *Parser) peekByte(b byte) bool {
 	if p.bsp == len(p.bs) {
 		p.fill()
@@ -882,7 +902,7 @@ loop:
 			tok = _Lit
 			break loop
 		case '?', '*', '+', '@', '!':
-			if p.peekByte('(') {
+			if p.isExprGlob() {
 				tok = _Lit
 				break loop
 			}