feat(yarn): add alias for `yarn why` (#10773)

* feat(yarn): add `yarn why`
* docs(yarn): update readme

Co-authored-by: Basia Józefowska <jozefowska.b@mpcreation.net>
Co-authored-by: Robby Russell <robby@planetargon.com>

plugins/yarn/README.md

@@ -55,3 +55,4 @@ zstyle ':omz:plugins:yarn' global-path no
 | yv    | `yarn version`                            | Update the version of your package                                            |
 | yw    | `yarn workspace`                          | Run a command within a single workspace.                                      |
 | yws   | `yarn workspaces`                         | Run a command within all defined workspaces.                                  |
+| yy    | `yarn why`                                | Show why a package has been installed, detailing which other packages depend on it |

plugins/yarn/yarn.plugin.zsh

@@ -43,3 +43,4 @@ alias yup="yarn upgrade"
 alias yv="yarn version"
 alias yw="yarn workspace"
 alias yws="yarn workspaces"
+alias yy="yarn why"