Update vc2 context and context usage.

[davidlehn]

• Switch from credentials-context to @digitalbazaar/credentials-context.
• Use all contexts from @digitalbazaar/credentials-context.
• Simplify context packages usage by using exported contexts directly.
• Improve credentials context mapping to and from versions.
  • Use explicit maps.
  • Expose a helper function.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.42%. Comparing base (203d698) to head (7a020cf).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## update-vc-2.0 #182 +/- ## ================================================= - Coverage 90.46% 90.42% -0.04% ================================================= Files 5 5 Lines 1059 1055 -4 ================================================= - Hits 958 954 -4 Misses 101 101 ``` | [Files](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar) | Coverage Δ | | |---|---|---| | [lib/contexts/index.js](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?src=pr&el=tree&filepath=lib%2Fcontexts%2Findex.js&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar#diff-bGliL2NvbnRleHRzL2luZGV4Lmpz) | `100.00% <100.00%> (ø)` | | | [lib/helpers.js](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?src=pr&el=tree&filepath=lib%2Fhelpers.js&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar#diff-bGliL2hlbHBlcnMuanM=) | `93.06% <100.00%> (+0.67%)` | :arrow_up: | | [lib/index.js](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?src=pr&el=tree&filepath=lib%2Findex.js&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar#diff-bGliL2luZGV4Lmpz) | `90.68% <100.00%> (-0.03%)` | :arrow_down: | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar). Last update [203d698...7a020cf](https://app.codecov.io/gh/digitalbazaar/vc/pull/182?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=digitalbazaar).

[dlongley]

Changelog needed.