There is no facility to Copy or Export/Import an effects stack

### Bug description

Unlike other parts of Audacity there is no facility to Copy or Export/Import an effects stack, which means that the user would have to recreate a carefully constructed and tried&tested effects stack by noting it down and recreating it manually.

You can:
a) import and export Macros
b) Effects have **Presets & Settings** where you can: save, deploy, import and export
c) you can store (Save) and retrieve (Load) Metadata settings

But you can't do this with an effects stack

### Steps to reproduce

1. get some audio
2. add some effects on the effects stack
3. tweak the parameters
4. try to save teh stack or export it
5. try to Save or Export
6. Observe: you can't

### Expected behavior

A facility to enable the user to save or export an effects stack with its settings so that it can be shared/replicated with other tracks and other projects

### Actual behavior

No facility to Copy or Export/Import an effects stack.

### Audacity Version

latest stable version (from audacityteam.org/download)

### Operating system

Windows

### Additional context

There is a kludgy workaround:

1. Duplicate the track with the required effects stack
2. delete the audio in the duplicated track
3. place your required audio in the empty track (paste od record)

This however has a nasty bug that will affect this:
The realtime effect settings of a duplicated track are not independent #4169

No source file or test is named. Start by comparing the existing Macro and Effects Presets & Settings workflows, then review the related realtime-effect duplication issue #4169. Done means users can copy or save an effects stack, export and import it, and preserve its settings for use on other tracks or projects.

Written by the indexing model from the issue text.