Custom variables in settings.json for easier profile configuration

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# Description of the new feature/enhancement
Let one define custom variables in settings.json for use with profiles.
E.g. define myCustomVariable for later use within commandline to avoid adding same long path to SSH key file for every profile name.
Would also be great to reuse current profile parameters such as "name" to connect to this hostname by stating $profile.name with SSH as shown in example.

# Proposed technical implementation details (optional)
```
"profiles":
{
"defaults":
{
"myCustomVariable": "C:\\Users\\USERNAME\\SSH_KEY_FILE_PATH.key"
},
"list":
[
{
"name": "HOSTNAME",
"commandline": "ssh -i $myCustomVariable $profile.name",
},
```

Start by reviewing Windows Terminal's settings.json profile configuration and the proposed defaults and list examples. Define the scope and expansion rules for custom variables and references such as $profile.name; the work is done when the example profile configuration is supported consistently.

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