Split "start work" button into two: one for starting work on an issue, one for creating a branch

[justinrobots]

Split the "Start Work" button into two:

Start work on an issue

• label: "Start work on an issue"
• flow: initiates flow for starting work on an issue and excludes basic create branch flow

Create a branch

• label: reuse "branch plus" icon from other similar flows
• button type: secondary button
• button width: just wide enough to display the icon
• flow: same flow as creating a branch from other places in GitLens, like from the graph