chore(build): Separate Feature Enabling Function

[byakuren-hijiri]

Closes #646

• I have updated CHANGELOG.md
• I have documented my contribution in Tact Docs: https://github.com/tact-lang/tact-docs/pull/PR-NUMBER
• I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• I have run all the tests locally and no test failure was reported
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[anton-trunov]

LGTM, but one suggestion though, let's start defining(centralizing) our API in index.ts (see, for instance, https://github.com/ton-org/ton-core/blob/main/src/index.ts). Maybe we can export enableFeatures and build in index.ts to start with. If we do it and add index.ts to Knip's entry points, then we won't have to ignore build.ts (which is the right thing to do).