New command, `spack change`, to change existing env specs (#31995)

If you have an environment like

```
$ cat spack.yaml
spack:
  specs: [openmpi@4.1.0+cuda]
```

this PR provides a new command `spack change` that you can use to adjust environment specs from the command line:

```
$ spack change openmpi~cuda
$ cat spack.yaml
spack:
  specs: [openmpi@4.1.0~cuda]
```

in other words, this allows you to tweak the details of environment specs from the command line.

Notes:

* This is only allowed for environments that do not define matrices
  * This is possible but not anticipated to be needed immediately
  * If this were done, it should probably only be done for "named"/not-anonymous specs (i.e. we can change `openmpi+cuda` but not spec like `+cuda` or `@4.0.1~cuda`)

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