tools: add find-inactive-collaborators.js

The plan is to eventually call this script with a scheduled GitHub
Action that could automatically open pull requests to move collaborators
to emeritus status after (for example) a year of inactivity.

Sample run:

```
$ node tools/find-inactive-collaborators.mjs '30 months ago'
864 authors have made commits since 30 months ago.
101 landers have landed commits since 30 months ago.
146 reviewers have approved landed commits since 30 months ago.
109 collaborators currently in the project.

Inactive collaborators:

Thomas Watson
$
```

tools/find-inactive-collaborators.mjs

@@ -0,0 +1,95 @@
+#!/usr/bin/env node
+
+// Identify inactive collaborators. "Inactive" is not quite right, as the things
+// this checks for are not the entirety of collaborator activities. Still, it is
+// a pretty good proxy. Feel free to suggest or implement further metrics.
+
+import cp from 'node:child_process';
+import fs from 'node:fs';
+import readline from 'node:readline';
+
+const SINCE = process.argv[2] || '6 months ago';
+
+async function runGitCommand(cmd, mapFn) {
+  const childProcess = cp.spawn('/bin/sh', ['-c', cmd], {
+    cwd: new URL('..', import.meta.url),
+    encoding: 'utf8',
+    stdio: ['inherit', 'pipe', 'inherit'],
+  });
+  const lines = readline.createInterface({
+    input: childProcess.stdout,
+  });
+  const errorHandler = new Promise(
+    (_, reject) => childProcess.on('error', reject)
+  );
+  const returnedSet = new Set();
+  await Promise.race([errorHandler, Promise.resolve()]);
+  for await (const line of lines) {
+    await Promise.race([errorHandler, Promise.resolve()]);
+    const val = mapFn(line);
+    if (val) {
+      returnedSet.add(val);
+    }
+  }
+  return Promise.race([errorHandler, Promise.resolve(returnedSet)]);
+}
+
+// Get all commit authors during the time period.
+const authors = await runGitCommand(
+  `git shortlog -n -s --since="${SINCE}"`,
+  (line) => line.trim().split('\t', 2)[1]
+);
+
+// Get all commit landers during the time period.
+const landers = await runGitCommand(
+  `git shortlog -n -s -c --since="${SINCE}"`,
+  (line) => line.trim().split('\t', 2)[1]
+);
+
+// Get all approving reviewers of landed commits during the time period.
+const approvingReviewers = await runGitCommand(
+  `git log --since="${SINCE}" | egrep "^    Reviewed-By: "`,
+  (line) => /^    Reviewed-By: ([^<]+)/.exec(line)[1].trim()
+);
+
+async function retrieveCollaboratorsFromReadme() {
+  const readmeText = readline.createInterface({
+    input: fs.createReadStream(new URL('../README.md', import.meta.url)),
+    crlfDelay: Infinity,
+  });
+  const returnedArray = [];
+  let processingCollaborators = false;
+  for await (const line of readmeText) {
+    const isCollaborator = processingCollaborators && line.length;
+    if (line === '### Collaborators') {
+      processingCollaborators = true;
+    }
+    if (line === '### Collaborator emeriti') {
+      processingCollaborators = false;
+      break;
+    }
+    if (line.startsWith('**') && isCollaborator) {
+      returnedArray.push(line.split('**', 2)[1].trim());
+    }
+  }
+  return returnedArray;
+}
+
+// Get list of current collaborators from README.md.
+const collaborators = await retrieveCollaboratorsFromReadme();
+
+console.log(`${authors.size.toLocaleString()} authors have made commits since ${SINCE}.`);
+console.log(`${landers.size.toLocaleString()} landers have landed commits since ${SINCE}.`);
+console.log(`${approvingReviewers.size.toLocaleString()} reviewers have approved landed commits since ${SINCE}.`);
+console.log(`${collaborators.length.toLocaleString()} collaborators currently in the project.`);
+
+const inactive = collaborators.filter((collaborator) =>
+  !authors.has(collaborator) &&
+  !landers.has(collaborator) &&
+  !approvingReviewers.has(collaborator)
+);
+
+if (inactive.length) {
+  console.log('\nInactive collaborators:');
+  console.log(inactive.join('\n'));
+}