Support LSX inline variables

Currently, it is possible to use variables inline in LSX:

```lua
lsx('<foo bar="${baz}" />', { baz = "buzz" })
```

But this syntax is a little janky, since the var is wrapped with double quotes AND the variable wrapper. Should be able to just directly use the variable, like in JSX:

```lua
lsx('<foo bar={baz} />', { baz = "buzz" })
```