npm package registry support for bin (#21372)

Fix #21303

npm package.json supports binary packaging:
https://docs.npmjs.com/cli/v8/configuring-npm/package-json#bin

the npm registry documents that the binary references will be attached
to the abbreviated version object:

https://github.com/npm/registry/blob/master/docs/responses/package-metadata.md#abbreviated-version-object

unfortunately their api documentation leaves this out:
https://github.com/npm/registry/blob/master/docs/responses/package-metadata.md#abbreviated-version-objectdoc

which is likely to be the reason this was left out in gitea's initial
implementation

this response is critical for npm to install the binary in the `.bin`
folder so as to be included on the users default bin path, resulting in
immediate access to any binaries provided by the package

modules/packages/npm/metadata.go

@@ -20,5 +20,6 @@ type Metadata struct {
 	DevelopmentDependencies map[string]string `json:"development_dependencies,omitempty"`
 	PeerDependencies        map[string]string `json:"peer_dependencies,omitempty"`
 	OptionalDependencies    map[string]string `json:"optional_dependencies,omitempty"`
+	Bin                     map[string]string `json:"bin,omitempty"`
 	Readme                  string            `json:"readme,omitempty"`
 }