A modal for asking values from user (= prompt)

[Taitava]

Discussed in #17

I might add more content to this issue - not new features, but things that I encounter that still needs to be done.

A non-exhaustive list of things to do:

• Preview shell command (a toggleable option in a Prompt): If on, while asking values from a user, display the shell command and bold the variable that is tied to the currently active input field. Maybe show just a bold variable name, or replace the variable name with the input value, to demonstrate concretely how the input will be inserted in the command.
• Enlarge prompt setting fields.
• Each prompt modal should have an HTML class attribute so that users can use CSS to customise each prompt modal individually.
• Fix crashes related to unselected target variables.
• Fix Uncaught (in promise) in PromptModal.ts in Execute button creation and in onClose() method. (Probably fixed).
• [ ] Make sure configuration migrations can add new fields to Prompts and PromptFields. Edit 2022-04-10: Moved to discussion Migrations: Better support for complex, nested configuration properties #198
• Shell command preview in prompts should use the partially parsed shell command when parsing custom variables, not a completely unparsed shell command (hard to express clearly). Then again, this is not critical, as it's only a preview and does not affect execution, but it's also easy to fix.
• Shorten tab label Operating systems & shells because now there's two new tabs that will take a lot of space (Preactions and Variables). Maybe just Environment would be good? Or Platform?
• Fix creating a new field in a Prompt does not take the field into use until the plugin is reloaded.
• Add debugLog() calls.