🚀 improve mounting behavior

[efectn]

Description

Fixes #2066 and adds some features to mounting feature 🚀

• Add OnMount hook that is fired when sub-app is mounted on a parent app.
• Add MountPath method to app. It returns mount prefix of sub-app. If app not mounteed, it returns empty string.
• Don't execute onRoute/OnGroup/OnName/OnGroupName hooks when to add routes of sub-apps to parent app. Return pathes with prefix for sub-app routes instead.
• Don't add routes in Mount. Add them in startupProcess. We're now able to Mount app before adding routes (supoorting nested mounting).
• Fix views when both of parent app and sub-app have different view engines

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• For new functionalities I follow the inspiration of the express js framework and built them similar in usage
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - https://github.com/gofiber/docs for https://docs.gofiber.io/
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I tried to make my code as fast as possible with as few allocations as possible

Commit formatting:

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[ReneWerner87]

some little adjustments needed

[ReneWerner87]

code looks good, but i have made a few comments which should clean up the code and make it easier to read and understand