Clarify allowed files and extensions when adding a package

[deelawn]

Description

Filtering exists for files uploaded when a new package is added. These filters are a combination of conditionals and regular expressions that exist in multiple places. The definition of filenames and extensions that are allowed should be better defined, unified, and put into a single location.

Here are the three locations I've found thus far:

gno/gnovm/pkg/gnolang/nodes.go

Lines 1110 to 1117 in 31a5f2e

	allowedFiles := []string{ // make case insensitive?
	"gno.mod",
	"LICENSE",
	"README.md",
	}
	allowedFileExtensions := []string{
	".gno",
	}

gno/tm2/pkg/std/memfile.go

Lines 42 to 46 in 31a5f2e

	var (
	rePkgName = regexp.MustCompile(`^[a-z][a-z0-9_]*$`)
	rePkgOrRlmPath = regexp.MustCompile(`^gno\.land\/(?:p|r)(?:\/_?[a-z]+[a-z0-9_]*)+$`)
	reFileName = regexp.MustCompile(`^([a-zA-Z0-9_]*\.[a-z0-9_\.]*|LICENSE|README)$`)
	)

gno/gno.land/pkg/gnoweb/gnoweb.go

Lines 114 to 117 in 31a5f2e

	app.Router.Handle("/r/{rlmname:[a-z][a-z0-9_]*(?:/[a-z][a-z0-9_]*)+}/{filename:(?:.*\\.(?:gno|md|txt|mod)$)?}", handlerRealmFile(logger, app, &cfg))
	app.Router.Handle("/r/{rlmname:[a-z][a-z0-9_]*(?:/[a-z][a-z0-9_]*)+}", handlerRealmMain(logger, app, &cfg))
	app.Router.Handle("/r/{rlmname:[a-z][a-z0-9_]*(?:/[a-z][a-z0-9_]*)+}:{querystr:.*}", handlerRealmRender(logger, app, &cfg))
	app.Router.Handle("/p/{filepath:.*}", handlerPackageFile(logger, app, &cfg))

Then there is also the addition of the license constant added as a temporary fix for exposing a LICENSE file via gnoweb:

gno/tm2/pkg/std/memfile.go

Lines 107 to 109 in 289785b

	case last == licenseName:
	return strings.Join(parts[:len(parts)-1], "/"), licenseName
	}