feat: `ResetContext` func to allow resetting initialized context

[kanstantsin-chernik]

The feature allows a context to be reset before reusing it in multiple go-routines (see the issue).

An alternative solution would be to always create a new context, which would be overkill for 99.9% of single-threaded use cases.

[joerdav]

My concern on this one is the unexpected effects that reseting the context could cause.

• Children
• Scripts
• CSS
• Once
• Security Nonces

We should either mitigate these in some way with this solution, or document the possible outcomes.