ci: add typo checking

typos is a powerful source code spell checker.

Adds a CI job that runs on every PR and
push to master (but can also be run manually with
workflow_dispatch) that checks for typos.

Adds a config file .typos.toml that deals with
false positives and only checks for top-level/one-level
.mediawiki and .md files.

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+# Contributing Guidelines
+
+Apart from following [BIP 2](./bip-0002.mediawiki),
+we do CI checks to ensure that the proposed BIPs do not have common typos.
+These checks are done using [`typos`](https://github.com/crate-ci/typos).
+To check for typos locally,
+install [`typos`](https://github.com/crate-ci/typos)
+and then run in the root directory:
+
+```bash
+typos
+```