v0.0-310-ga9d1b64: Lint: Make acceptance of anonymous nested structs configurable.
This adds 'allow_anonymous_nested' configuration to the 'typedef-structs-unions'
rule to allow not to require a typedef for inner structs.
In the course of that, implement SyntaxTreeContext::IsInsideStartingFrom()
which starts looking backward the stack starting with an offset.
(As a side effect, make IsInside() to start looking from top of stack
instead of bottom, which is conceptually more aligned of what we expect,
even though it makes only a difference in this case (offset 0))
Fixes #128
PiperOrigin-RevId: 302738764