New "spaceless" alternative mode where whitespace is opt-in with `{#+`.

Current situation

Currently, adding a - to a twig tag causes whitespace removal.
E.g. {#- some comment #}, here whitespace is removed before but not after the comment.

Without the -, a single whitespace between twig tags is removed, but a double line break \n\n is not.

Proposal

I would like to propose a new mechanism that turns this around.

We introduce a new mode-switching twig tag {% spaceremoved %}, which would typically be placed at the top of a template.
(I would call it "spaceless" but that term now has a history. Open to suggestions.)

This allows to opt-in to this mechanism per template or per snippet, thus avoiding BC problems.

When this mode is enabled:

• A twig tag without - is treated as if it had -. E.g. {# some comment #} is treated like {#- some comment #}.
• A twig tag with + is treated like a regular tag without -. So, {#+ some comment +#} is treated like {# some comment #}.

Example

{% spaceremoved %} 
hel {# remove space before and after #} lo {#+ keep space before #} world

This is equivalent to

hel {#- remove space before and after -#} lo {# keep space before -#} world

which prints as

hello world

See also

I found

• #4442, which discusses the same problem, but does not propose this specific solution.

No implementation files or tests are named in the issue. Start by reviewing the existing whitespace-control behavior and the related discussion in #4442, then clarify the syntax and backward-compatibility requirements for the proposed mode. Done means the behavior and implementation scope are agreed and covered by suitable tests.