chore(gnoweb): cleanup iteration on gnoweb

[gfanton]

depends on #3366

This PR cleans up, documents, and reorganizes the gnoweb package, which was recently revamped:

• Refactored the code for better readability and structure, and added enhanced comments.
• Enhanced existing test cases:
  • Added new test cases for assets in app_test.go.
  • Included a new test rule in the Makefile.
  • Created new tests for WebHandler in handler_test.go.
• Improved file and directory handling methods in handler.go.

[Gno2D2]

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[codecov]

Codecov Report

Attention: Patch coverage is 68.14159% with 108 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gno.land/pkg/gnoweb/webclient_html.go	57.94%	35 Missing and 10 partials ⚠️
gno.land/pkg/gnoweb/handler.go	76.10%	24 Missing and 3 partials ⚠️
gno.land/pkg/gnoweb/format.go	44.82%	13 Missing and 3 partials ⚠️
gno.land/pkg/gnoweb/webclient_mock.go	78.26%	7 Missing and 3 partials ⚠️
gno.land/pkg/gnoweb/app.go	75.00%	6 Missing and 3 partials ⚠️
gno.land/cmd/gnoweb/main.go	83.33%	1 Missing ⚠️

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[thehowl]

generally lgtm