[ios, macos] Add NSExpression convenience constructors and helper methods. (#11278)
* [ios, macos] Refactor convinience expression initializers.
# Conflicts:
# platform/darwin/src/NSExpression+MGLAdditions.h
# platform/darwin/src/NSExpression+MGLAdditions.mm
# platform/ios/app/MBXViewController.m
* [ios, macos] Add MGL_MATCH convenience method.
# Conflicts:
# platform/darwin/src/NSExpression+MGLAdditions.h
# platform/darwin/src/NSExpression+MGLAdditions.mm
# platform/darwin/test/MGLExpressionTests.mm
* [ios, macos] Add Swift friendly convenience methods name syntax..
# Conflicts:
# platform/darwin/src/NSExpression+MGLAdditions.h
* [ios, macos] Update runtime styling examples.
# Conflicts:
# platform/ios/app/MBXViewController.m
* [ios, macos] Add NSExpression variable expressions properties.
* [ios, macos] Clarify convenience initializer names.
* [ios, macos] Update Style Authors guides.
* [ios, macos] Fix cherry-pick merge.
* [ios, macos] Update convenience initializers documentation.
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