<Textfield/> does not handle input$readOnly attribute correctly.

[thubalek]

Describe the bug
<Textfield/> does not handle input$readOnly attribute correctly. Tested with SMUI version 7.0.0

To Reproduce
Steps to reproduce the behavior:

1. Use <TextField ... input$readOnly="{true}"/>
2. Use <TextField ... input$readOnly="{false}"/>

Idea for this code comes from #280 (comment)

Expected behavior
In first case there should be HTML code in style
<input ... readonly/>

In second case there should be attribute readonly omitted but it is rendered as

<input type="text" readonly="false"/>

that is wrong as readonly="false" is is interpreted by browser as readonly="true".

See https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#boolean-attribute and https://html.spec.whatwg.org/multipage/input.html#attr-input-readonly for explanation.

Additional comments
The problem is that following code does not work.

<script>
  import Textfield from "@smui/textfield";
  let ro : boolean = false;
</script>

<TextField ... input$readonly="{ro}"/>