feat(b-table, b-table-lite): switch slot name syntax to use round brackets instead of square brackets

[tmorehouse]

Describe the PR

Switch cell[...], head[...] and foot[...] slot name syntax to cell(...), head(...) and foot(...) syntax, to prevent user confusion with dynamic slot name syntax.

Since we are breaking the slot names in 2.0.0 stable, this is the time to make the change.

PR checklist

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

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

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

• No
• Yes (please describe)

Similar to the PR that changed the slot names to lower-case and square brackets

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 #3986 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev    #3986   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files         238      238           
  Lines        4449     4449           
  Branches     1250     1250           
=======================================
  Hits         4443     4443           
  Misses          5        5           
  Partials        1        1

Impacted Files	Coverage Δ
src/components/table/helpers/mixin-thead.js	100% <100%> (ø)	⬆️
src/components/table/helpers/mixin-tbody-row.js	100% <100%> (ø)	⬆️

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