fix(form-file): Add prop to allow customization of browse button text. fixes #2143

[tmorehouse]

Description of Pull Request:

Adds new prop to customize browse button text.

Includes temporary CSS until Bootstrap V4.2 is released

fixes #2143

PR checklist:

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

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe:

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

• Yes
• No

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 #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR 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.
• PR titles should following the Conventional Commits naming convention (i.e. "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos", "chore: fix typo in docs", 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 #2168 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #2168   +/-   ##
=======================================
  Coverage   66.01%   66.01%           
=======================================
  Files         158      158           
  Lines        2998     2998           
  Branches      832      832           
=======================================
  Hits         1979     1979           
  Misses        739      739           
  Partials      280      280

Impacted Files	Coverage Δ
src/components/form-file/form-file.js	10.66% <ø> (ø)	⬆️

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