add snippets of unsupported constructions for dockerfile, go, js and rb

fixtures/dockerfile-

@@ -0,0 +1,8 @@
+# The grammar defines the user as a method, which is a bug.
+RUN \
+	user
+
+# There is no scope for backslash.
+RUN \
+	echo \
+	echo

fixtures/go-

@@ -1,3 +1,8 @@
 // There is no scope for the comparable in the Go grammar.
 // https://github.com/worlpaker/go-syntax/issues/20/
 func f[K comparable, V any]()
+
+// There is no scope for a property whose name may resemble a primitive type.
+struct {
+	int int
+}{}

fixtures/js-

@@ -8,3 +8,10 @@ const of = ""
 // It is not a part of the core javascript grammar.
 // https://github.com/0x00000001A/es6-string-html/issues/51/
 /*css*/``
+
+// I would like to highlight the text inside JSX tags in the same way as it is
+// highlighted in regular HTML tags.
+<div>txt</div>
+
+// I would like to have a scope that includes everything after the as keyword.
+let a = a as number | Error | string

fixtures/rb-

@@ -0,0 +1,3 @@
+# There are no scopes for number separators.
+1_0
+1.0