[FEATURE] ViewHelpers to set/get TSFE-registers

Allows accessing values previously set in TypoScript
via LOAD_REGISTER, or to set values to access elsewhere
from the registers.
Merge pull request #639 from neufeind/feature/register-VH

[FEATURE] ViewHelpers to set/get TSFE-registers
Merge branch 'master' into development
[TASK] Regenerate XSD
[TASK] Regenerate XSD
Merge branch 'master' into staging
Merge branch 'staging' into development
[FEATURE] Argument $single on Iterator / Extract ViewHelper

When used, returns the first value instead of always returning all values. Use when result is expected to contain only one value, and avoid having to chain -> v:iterator.first() after .extract() to get to the actual value.
[BUGFIX] Generate language links equally for current/others

Close: #672
[DOC] Escape constant names in Iterator / Sort order argument

Closes: #668
[TASK] Fix composer.json version for 2.0.1
[TASK] Deprecate "arguments" on Asset in favor of "variables"
[TASK] Fix composer.json version for 2.0.1
[BUGFIX] Incorrect template variable container key
Merge pull request #675 from bjo3rnf/containerkey

[BUGFIX] Incorrect template variable container key
[BUGFIX] Avoid 6.2-only array merge utility on 6.1
Merge branch 'development' into staging
[TASK] Raise composer version to 2.0.2
[TER] 2.0.2