rename vue attribute macros to @if, @else, @elseif and @for

[hhaensel]

Currently we have @iif, @els, @elsiifand @recur defined due to name clashes with Julia's reserved words.
From version 1.10 on one can get around that by defining a macro var"if".
However in order to be compatible, macro aliasing is a better way. So we keep the old definitions and alias them like

const var"@if" = var"@iif"

We even get the docstring right if we use it in the REPL:

help?> @if
  @if(expr)

  Generates v-if Vue.js code using expr as the condition. https://vuejs.org/v2/api/#v-if (https://vuejs.org/v2/api/#v-if)

  Example
  –––––––

  julia> span("Bad stuff's about to happen", class="warning", @if(:warning))
  "<span class="warning" v-if='warning'>Bad stuff's about to happen</span>"