Limit nesting depth by default to guard against stack overflow

[spyoungtech]

This change adds a limit to the nesting depth by default to guard against stack overflow in cases where arrays and/or objects are deeply nested.

Additionally, a new feature, unlimited_depth has been added (disabled by default) as an easy way for users to bypass this limit.

JSON5Parser structs also have a new constructor with_max_depth to allow a parser to be constructed with a user-specified max depth.

The default limit aims to prevent stack overflows in most normal circumstances on tier 1 operating systems. Some targets with extremely limited stack space may still experience stack overflows. The limit is adjusted based on operating system and the presence of the debug_assertions configuration, intended to allow higher default limits for release builds.

At time of writing, the default limit is as follows:

Operating System	Depth limit with debug_assertions	Depth limit without debug_assersions
Windows	700	2000
All other OS	1000	3000

[spyoungtech]

Because parse_unary is also implemented recursively, the depth limit also applies to unary signs.