feat: support environment variable references in maven settings server credentials

[jekkel]

References of the form ${env.NAME} are now substituted with the corresponding env variable value, if it exists. Otherwise, no substitution occurs.

fixes #130

[github-actions]

Coverage after merging feature/env-substitution-for-credentials into main will be

81.84%	▴ +0.24%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
lib
content-source-factory.js	92.86%	50%	100%	100%	31
extension.js	86.67%	66.67%	100%	92.86%	55–56, 56, 56–57, 70
maven-client.js	47.58%	30%	66.67%	51.76%	102–103, 103, 103–105, 108, 112–113, 135, 141–143, 143, 143–144, 146, 154, 154, 183, 194–195, 201–204, 212–213, 213, 213–215, 217–219, 223–224, 224, 224–225, 227–228, 228, 228–229, 231, 237–238, 34–35, 37, 37, 46, 50, 52, 64–65, 91–93, 98–99
maven-content-source.js	77.27%	50%	80%	85.11%	106–107, 115–116, 118–119, 121–122, 135–136, 49–50, 73, 75
maven-files.js	88.49%	71.11%	100%	96.39%	164, 168, 168–169, 169, 169, 171, 171, 171–172, 172, 172, 36, 38, 58–59
maven-types.js	81.13%	71.70%	90%	90.70%	118, 120, 26, 26, 36–37, 78–79, 86–87, 93, 93, 93, 93, 93, 93, 93, 93, 93
version-selector.js	82.93%	81.25%	100%	82.61%	30, 30, 32–33, 4–6
versioning.js	86.67%	89.19%	80%	87.30%	105, 110–111, 121–122, 124–125, 143, 28, 35, 84–85