add js_attr(x) for setting julia expressions as js attributes, fix JSONText in Plots

[hhaensel]

Typical use case

using StippleUI

 quasar(
    :btn__toggle,
    fieldname = :btn_value,
    options = js_attr([opts(label = "Off", value = false), opts(label = "On", value = true)])
 )

# "<q-btn-toggle v-model=\"btn_value\" :options=\"[{'value':false,'label':'Off'}, {'value':true,'label':'On'}]\"></q-btn-toggle>"

Furthermore, a final fix for JSONText was necessary...