feat: add getKeypairFromFile() and getKeypairFromEnvironment()

Plus tests and docs etc.

.github/workflows/tests.yaml

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+# From https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-nodejs#using-the-nodejs-starter-workflow
+name: Node.js and Solana CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [20.x]
+
+    steps:
+      - uses: actions/checkout@v3
+      # Install Solana
+      # From https://docs.solana.com/cli/install-solana-cli-tools
+      - run: sh -c "$(curl -sSfL https://release.solana.com/stable/install)"
+
+      - name: Add to PATH
+        shell: bash
+        run: |
+          echo "/home/runner/.local/share/solana/install/active_release/bin" >> $GITHUB_PATH
+
+      # Install everything
+      - run: npm ci
+
+      - name: Check Solana keygen is installed
+        run: which solana-keygen
+
+      # Run tests
+      - run: npm test

.prettierrc.json

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+{}

README.md

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+# Solana node helpers
+
+This package contains node-js specific Solana helper functions. Unlike `@solana/web3.js` - which works in both the browser and node.js - the `node-helpers` package is just for node.js functions, for example functions that load files.
+
+## getKeypairFromFile
+
+Gets a keypair from a file - the format must be the same as [Solana CLI](https://docs.solana.com/wallet-guide/file-system-wallet) uses, ie, a JSON array of numbers:
+
+To load the default keypair `~/.config/solana/id.json`, just run:
+
+```
+const keyPair = await Keypair.fromFile();
+```
+
+or to load a specific file:
+
+```
+const keyPair = await Keypair.fromFile('somefile.json');
+```
+
+or using home dir expansion:
+
+```
+const keyPair = await Keypair.fromFile('~/code/solana/demos/steve.json');
+```
+
+## getKeypairFromEnvironment
+
+Gets a keypair from a secret key stored in an environment variable. This is typically used to load secret keys from [env files](https://stackoverflow.com/questions/68267862/what-is-an-env-or-dotenv-file-exactly).
+
+### Secret key format
+
+Secret keys can be in either the more compact base58 format (`base58.encode(randomKeypair.secretKey);`), like:
+
+```
+# A random secret key for demo purposes
+SECRET_KEY=QqKYBnj5mcgUsS4vrCeyMczbTyV1SMrr7SjSAPj7JGFtxfrgD8AWU8NciwHNCbmkscbvj4HdeEen42GDBSHCj1N
+```
+
+Or the longer, array of numbers format `JSON.stringify(Object.values(randomKeypair.secretKey));`:
+
+```
+# A random secret key for demo purposes
+SECRET_KEY=[112,222,91,246,55,109,221,4,23,148,251,127,212,180,44,249,182,139,18,13,209,208,6,7,193,210,186,249,148,237,237,1,70,118,1,153,238,134,239,75,187,96,101,138,147,130,181,71,22,82,44,217,194,122,59,208,134,119,98,53,136,108,44,105]
+```
+
+## Development
+
+To run tests
+
+```
+npm run test
+```
+
+The tests use the [node native test runner](https://blog.logrocket.com/exploring-node-js-native-test-runner/).