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'

package.json

@@ -27,6 +27,7 @@
     "build:js": "./scripts/build-js.sh",
     "build:release": "pezkuwi-ci-ghact-build",
     "build:rollup": "pezkuwi-exec-rollup --config",
+    "build:scure-sr25519": "node node_modules/typescript/bin/tsc -p packages/scure-sr25519/tsconfig.json && node node_modules/typescript/bin/tsc -p packages/scure-sr25519/tsconfig.cjs.json",
     "build:wasm": "./scripts/build.sh",
     "clean": "./scripts/clean.sh",
     "deno": "yarn deno:build && yarn deno:check",