[TwigComponent] allow attributes to be prefixed with `@`

[kbond]

Q	A
Bug fix?	yes
New feature?	no
Issues	Fix (partial) #1839
License	MIT

Allow attributes to be prefixed with @ (like @click="!open").

[kbond]

Currently, this PR only adds a failing test demonstrating the problem reported in #1839. I could use some help to fix!

[smnandre]

In TwigPreLexer , i guess consumeAttributeName should use another regexp than the one in consumeComponentName

But we need a lot of tests for this, as they cannot be class props !