Add join-keepNamed option flag to optimize CLI

[jo-chemla]

Is your feature request related to a problem? Please describe.
The current optimize CLi lacks the ability to define join options like keepNamed and keepMeshes. It only allows for a bool flag to enable or not join

Describe the solution you'd like
It could be useful to expose these flags to the optimize CLI.

Describe alternatives you've considered
Add the flag here in the cli.js script

glTF-Transform/packages/cli/src/cli.ts

Line 335 in ec58e13

.option('--join <bool>', 'Join meshes and reduce draw calls. Requires `--flatten`.', {

[jo-chemla]

Just did a small PR to add these --join-keep-named and --join-keep-meshes flags: #1551