fix(b-modal): delay initially open modal via nextTick when using v-model or visible prop

[tmorehouse]

Describe the PR

When modal is set to be initially open on mount, delay the show in a $nextTick() to help ensure DOM is fully loaded

Addresses #3312 (comment)

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #3320 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev    #3320   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files         218      218           
  Lines        4142     4142           
  Branches     1197     1197           
=======================================
  Hits         4110     4110           
  Misses         26       26           
  Partials        6        6

Impacted Files	Coverage Δ
src/components/modal/modal.js	99.52% <100%> (ø)	⬆️

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