This commit adds a full example for the IfLoadedDirective and moves
the code to the StackBlitz playground for structural directives.
Fixes#40739, #37119
PR Close#46425
tweak the current structural directives guide (currently mainly targeted
at the creation of custom structural directives) so that is more generic
and a point of reference for structural directives in general
this also includes the re-addition of the one-per-element section
removed in PR #40015resolves#44786
PR Close#44895
add types to all the buttons in the angular.io examples to encourage
the best practice of always including a type per button (regardless
to whether it is in a form or now)
PR Close#44557
BREAKING CHANGE: the compiler option `enableLegacyTemplate` is now disabled by default as the `<template>` element has been deprecated since v4. Use `<ng-template>` instead. The option `enableLegacyTemplate` and the `<template>` element will both be removed in Angular v6.
PR Close#18756