[6.x] Prevent variable conflicts with wildcard tags

[duncanmcclean]

Currently, when you do something like this in your template:

{{ nav:footer }}
    <!-- -->
{{ /nav:footer }}

Statamic will check if a footer variable exists in the tag's context and use its value, otherwise it'll fallback to the literal string, footer.

This behaviour is a little problematic, for example if you have a global called footer, but you also have a nav called footer. It would end up passing your augmented global as the handle for the nav, which obviously won't work.

This issue only affects a couple of our tags:

• {{ session }}
• {{ cookie }}
• {{ nav }}
• {{ redirect }}

We're doing similar things in the {{ dump }}, {{ asset }}, {{ get_content }} and {{ glide }} tags but the behaviour makes sense in those cases.

Fixes #10612
Fixes #10643