Improve Plots Wizard

[julieg18]

• Make the reasoning behind the first file selector more obvious. Consolidate our "Create Custom Plot" and "Create Plot" commands, having a single "Create Plot" command with an extra quick pick. (#4664 (review))
• Add an action into the plots webview (add a "placeholder" plot that is button for adding plots (breaks drag and drop) add a plus button to the Data Series inputs)
• Look into using WorkspaceEdit API (#4628 (comment)). Could be used but wouldn't simplify anything.
• Look into allowing multiple files being selected for a plot (https://dvc.org/doc/command-reference/plots/show#sourcing-data-from-different-files)
• Add option for choosing the plot title
• Allow multiple metrics to be selected for y
• Allow multiple metrics to be selected for x (#4787 (comment))
• Group tests in /pipeline/quickPick.test.ts. (decided on leaving file as is)

Followup to #4628

[dberenbaum]

@julieg18 Was there at some point an extra plots pane in the circled area with a + to add a new plot? I'm wondering if we can make the plots wizard more obvious.

[julieg18]

@julieg18 Was there at some point an extra plots pane in the circled area with a + to add a new plot? I'm wondering if we can make the plots wizard more obvious.

Yes, we wanted to do this originally, but adding one breaks our plot drag-and-drop code. It is most likely doable but would complicate our drag-and-drop code (which is already pretty complex as is) so we decided to not do this for now.