Add size param to splitPane action, split-pane subcommand (#8543)
## Summary of the Pull Request
Adds a `size` parameter to `splitPane`. This takes a `float`, and specifies the portion of the parent pane that should be used to create the new one.
This also adds the param to the `split-pane` subcommand.
### Examples
| commandline | result |
| -- | -- |
| `wt ; sp -s .25` |  |
| `wt ; sp -s .8` |  |
| `wt ; sp -s .8 ; sp -H -s .3` |  |
## PR Checklist
* [x] Closes #6298
* [x] I work here
* [x] Tests added/passed
* [x] Docs PR: MicrosoftDocs/terminal#208
## Detailed Description of the Pull Request / Additional comments
I went with `size`, `--size,-s` rather than `percent`, because the arg is the (0,1) version of the size, not the (0%,100%) version.
## Validation Steps Performed
Added actions, played with the commandline, ran tests
4f46129cb4
