chore(scure-sr25519): add the source lib/ is built from (#17)

The package shipped only compiled output, and no repository held its
source. lib/ is @scure/sr25519 0.2.0 with three edits made directly in
the compiled files: signing context 'substrate' -> 'bizinikiwi' and the
two header comment lines. index.ts is upstream's 0.2.0 source with the
same edits; compiled with its own tsconfig it reproduces lib/*.js and
lib/*.d.ts byte for byte. Only the source maps change, since the
hand-edited ones still described upstream's file.

- index.ts, tsconfig.json, tsconfig.cjs.json, upstream MIT LICENSE
- yarn build:scure-sr25519 rebuilds lib/
- scripts/check-scure-sr25519.sh, run in CI, fails when lib/ is not a
  build of index.ts
- README: what differs from upstream, how to build; it said the context
  was 'bizinikiwi' instead of 'bizinikiwi'

.github/workflows/ci.yml

@@ -37,6 +37,11 @@ jobs:
       - name: Lint
         run: yarn lint
 
+      # scure-sr25519 ships committed compiled output; it must be a build of
+      # the index.ts beside it, not hand-edited JavaScript.
+      - name: Check scure-sr25519 is built from its source
+        run: ./scripts/check-scure-sr25519.sh
+
   test:
     runs-on: ubuntu-latest
     steps: