Add eXist-db.app v3.0RC1

[joewiz]

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
  Note: Much as @alyssais did in Add Slack.app v2.2.6-beta1 #2578, I opted to use "rc" as the suffix rather than a version number (like 3). This is because this recipe is intended for the current release candidate (eXist's convention for beta/preview releases), regardless of the current major/minor version number.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked there are no open pull requests for the same cask.
• Checked that the cask was not already refused in closed issues.

[joewiz]

Thank you for your review and merge! I am happy with exist-db3 as opposed to exist-db-rc.

Two questions:

1. When eXist-db v3.0 is released and I update the main cask to use v3.0 instead of v2.2, should I delete this cask, or keep it for people who explicitly want v3.x as opposed to "the current version" (i.e., even in the future after a hypothetical v4.x is released)?

2. Since 2.2 still has a large install base, I imagine some users would still want that even once v3.0 is released. This suggests that adding a "eXist-db2" cask would make sense, until v2.x is considered obsolete, right?

[vitorgalvao]

When 3 becomes stable, this cask should be removed.

One for version 2 should not be added. Later if it becomes necessary, someone else will do it.

[joewiz]

Got it. Thanks for explaining; I think I'm starting to understand the approach.