Qute - support optional end tags for sections

[mkouba]

Description

In some cases, it might be useful to support optional end tags. A typical example is the {#let} section.

Currently, the end tag is mandatory and defines the scope of the "local variables":

{#let foo=true}
  // foo can be used here
{/let}
// foo cannot be used here

However, it might be practical to make the end tag optional and bind the section to the parent:

{#let foo=true}
// foo can be used here

// synthetic {/let} added automatically

// foo cannot be used here
{#if baz}
{#let foo=true}
// foo can be used here

// synthetic {/let} added automatically
{/if}
// foo cannot be used here

By default, a missing end tag still results in a parser error. The strategy used will be controlled by the relevant SectionHelperFactory.

Implementation ideas

No response