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` | ![image](https://user-images.githubusercontent.com/18356694/101784317-fb595680-3ac0-11eb-8248-782dc61957cf.png) |
| `wt ; sp -s .8` | ![image](https://user-images.githubusercontent.com/18356694/101784442-20e66000-3ac1-11eb-8f9b-fb45a73c9334.png) |
| `wt ; sp -s .8 ; sp -H -s .3` | ![image](https://user-images.githubusercontent.com/18356694/101784552-470c0000-3ac1-11eb-9deb-df37aaa36f01.png) |
## 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