Add support for SubRouter

[tigerwill90]

Summary

This PR introduces several significant features and improvements to the Fox router:

• SubRouter Support: Enable nested routing by mounting sub-routers with pattern prefixes, allowing modular route composition
• Optional Named Wildcards (+{param}): New catch-all syntax that matches any sequence of characters including empty strings, useful for optional path suffixes
• Method-less Routes (fox.MethodAny): Register routes that match any HTTP method, with specific methods taking precedence (this is only a transition, next PR will use matchers for matching methods)
• Context API Refactoring: Replace Context interface with concrete *Context type (which also was a requirement for the SubRouter feature). This should slightly improve the performance.

Example

api := fox.MustNew()
api.MustHandle(http.MethodGet, "/", func(c *fox.Context) {
	_ = c.String(http.StatusOK, "Welcome")
})
api.MustHandle(http.MethodGet, "/users/{id}", func(c *fox.Context) {
	_ = c.String(http.StatusOK, fmt.Sprintf("Hello %s\n", c.Param("id")))
})

f := fox.MustNew(fox.DefaultOptions())
route, err := f.NewSubRouter("/api+{any}", api)
if err != nil {
	panic(err)
}
if err := f.HandleRoute(MethodAny, route); err != nil {
	panic(err)
}

[codecov]

Codecov Report

❌ Patch coverage is 82.29572% with 91 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.89%. Comparing base (f4c530a) to head (a211c2e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
fox.go	85.66%	34 Missing and 11 partials ⚠️
error.go	12.50%	14 Missing ⚠️
txn.go	29.41%	12 Missing ⚠️
tree.go	79.48%	5 Missing and 3 partials ⚠️
context.go	88.52%	4 Missing and 3 partials ⚠️
route.go	33.33%	4 Missing ⚠️
recovery.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   89.98%   88.89%   -1.09%     
==========================================
  Files          25       25              
  Lines        3836     4135     +299     
==========================================
+ Hits         3452     3676     +224     
- Misses        296      359      +63     
- Partials       88      100      +12     

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