fix(b-form-file): fix prop type checking for value prop

[tmorehouse]

Describe the PR

When creating the new props docs and web-types (for yet to be released v2.0.3), the value type was changed to [File, Array] from Any (undefined) for better prop docs generation.

The resetting of the file control can be done by setting to null, and empty array, or an empty string, and when doing so Vue would emit an prop type check warning (when using empty string to reset the input).

This adds a deprecation message to have users switch to setting to null instead of empty string, and uses a custom validator to check the prop type/value

PR checklist

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

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe) Minor fix/refactor

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 #4168 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4168      +/-   ##
==========================================
+ Coverage   99.86%   99.86%   +<.01%     
==========================================
  Files         237      237              
  Lines        4493     4495       +2     
  Branches     1258     1258              
==========================================
+ Hits         4487     4489       +2     
  Misses          5        5              
  Partials        1        1

Impacted Files	Coverage Δ
src/utils/inspect.js	100% <100%> (ø)	⬆️
src/components/form-file/form-file.js	96% <100%> (+0.08%)	⬆️

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