feat(b-img, b-img-lazy): add support for `srcset` and `sizes` props (closes #4348)

[tmorehouse]

Describe the PR

Add new optional props srcset and sizes (not supported on IE 11 though)

Props will either accept a string or array of strings

For b-img-lazy, the values of srcset and sizes will only be applied when the image is in view. src is still required for fallback image.

Closes #4348

Caveat: vue-loader does not support the processing of srcset (multiple URLs), so project relative URLs must be required, but one has to be cautious of images being converted to data URIs as one can exceed the maximum supported attribute value length.

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)
• Includes any needed TypeScript declaration file updates
• 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 #4350 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4350      +/-   ##
==========================================
+ Coverage   99.91%   99.91%   +<.01%     
==========================================
  Files         238      238              
  Lines        4577     4585       +8     
  Branches     1284     1286       +2     
==========================================
+ Hits         4573     4581       +8     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
src/components/image/img-lazy.js	100% <100%> (ø)	⬆️
src/components/image/img.js	100% <100%> (ø)	⬆️

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