build: add codecov.yml (#565)

## PR Checklist

- [x] Addresses an existing open issue: fixes #564
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/template-typescript-node-package/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/template-typescript-node-package/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Adds a [`codecov.yml` config
file](https://docs.codecov.com/docs/codecov-yaml) with settings from
https://docs.codecov.com/docs/notifications#preventing-notifications-until-after-n-builds.

Author: JoshuaKGoldberg

codecov.yml

@@ -0,0 +1,5 @@
+codecov:
+  notify:
+    after_n_builds: 3
+comment:
+  after_n_builds: 3

src/hydrate/steps/clearUnnecessaryFiles.ts

@@ -12,6 +12,7 @@ const globPaths = [
 	".npmignore",
 	"CODE_OF_CONDUCT.md",
 	"CONTRIBUTING.md",
+	"codecov.yml",
 	"DEVELOPMENT.md",
 	"dist",
 	"lib",