feat(env-var): add build.availity.com support

[jordan-a-young]

Summary

Adds build.availity.com (and its non-prod variants) as a recognized Availity portal URL in @availity/env-var.

Changes

src/index.ts

• Updated all 4 regexes to include build alongside apps and essentials:
  • DEFAULT_ENVIRONMENTS for test, qa, and prod
  • DEFAULT_SPECIFIC_ENVIRONMENTS for specific slug extraction
• Updated EnvOpts JSDoc to document build.availity.com as a prod URL

src/index.test.ts

• Added build URL variants to all existing test/qa/prod coverage in both getCurrentEnv/envVar and getSpecificEnv suites
• Also filled pre-existing gaps: missing essentials variants for numbered test/qa envs (e.g. t01-essentials, q01-essentials)

README.md

• Added a supported hostnames table documenting all URL patterns per environment

Supported URLs (after this change)

Environment	Hostnames
test	test-{apps|essentials|build}, t{NN}-{apps|essentials|build}
qa	qa-{apps|essentials|build}, qap-{apps|essentials|build}, q{NN}-{apps|essentials|build}
prod	apps, essentials, build

Testing

124 tests passing (up from 102 before this PR).