Insert a Variable button on variable editor

[MJRichardson]

Similar to the Insert Variable button when editing the deployment process, the same should be available when editing variables.

At the least in the editor dialog:

But also in the inline editor if this can be done nicely:

Most recent request: https://help.octopus.com/t/variable-editor-cannot-select-from-a-list-of-system-tenant-etc-variables/23464

There have been issues created for this in the past (and it may have even been implemented at some point):
#522
#522

[MarkSiedle]

@MJRichardson This issue may be more complicated than just adding the existing list of project-variables like we have on steps-bindings. Customers were struggling to make the connection with our System Variables as well (things like OctopusPrintVariables or the Azure custom bundling variable flag, which are not in that variables list).

So we're hoping by the end of this issue, customers have a really intuitive (and easily-discoverable) way of setting system variables. If you agree, it'd be great if usability could be involved here so we cover a bunch of scenarios that our team have been considering here.