parser: check interface methods name (fix #14217) (#14218)

vlib/v/checker/tests/interface_method_name_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/interface_method_name_err.vv:2:2: error: method name `Fizz` cannot contain uppercase letters, use snake_case instead
+    1 | interface Foo {
+    2 |     Fizz()
+      |     ~~~~~~
+    3 | }
+    4 |

vlib/v/checker/tests/interface_method_name_err.vv

@@ -0,0 +1,6 @@
+interface Foo {
+	Fizz()
+}
+
+fn main() {
+}

vlib/v/parser/struct.v

@@ -516,7 +516,8 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	mut mut_pos := -1
 	mut ifaces := []ast.InterfaceEmbedding{}
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
-		if p.tok.kind == .name && p.tok.lit.len > 0 && p.tok.lit[0].is_capital() {
+		if p.tok.kind == .name && p.tok.lit.len > 0 && p.tok.lit[0].is_capital()
+			&& p.peek_tok.kind != .lpar {
 			iface_pos := p.tok.pos()
 			mut iface_name := p.tok.lit
 			iface_type := p.parse_type()
@@ -584,10 +585,6 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 				p.error_with_pos('duplicate method `$name`', method_start_pos)
 				return ast.InterfaceDecl{}
 			}
-			if language == .v && util.contains_capital(name) {
-				p.error('interface methods cannot contain uppercase letters, use snake_case instead')
-				return ast.InterfaceDecl{}
-			}
 			// field_names << name
 			args2, _, is_variadic := p.fn_args() // TODO merge ast.Param and ast.Arg to avoid this
 			mut args := [