Text domain corrections

https://developer.wordpress.org/plugins/internationalization/how-to-internationalize-your-plugin/ says regarding declaring a text domain:

> After WordPress 4.6 came out, the Text Domain header is no longer required if it’s the same as the plugin slug. It’s now the default value.

However, this plugin does not declare a text domain, and uses multiple text domains.

In the start and end of this quoted section, we use `'pym_shortcode'`

https://github.com/INN/pym-shortcode/blob/7ad2ca8d694e030944d37123622e4820f19d4f17/inc/class-pymsrc-output.php#L123-L138

Elsewhere in the plugin, we use `'pym-embeds'`:

https://github.com/INN/pym-shortcode/blob/b62b061ba3a0438878b110b18f0c1b1b59faf76d/inc/settings-page.php#L59-L60

https://github.com/INN/pym-shortcode/blob/a8666141cbb12876e36de77fc96578e680e3ef00/inc/info-page.php#L20-L21

Neither of those matches the [.org plugin slug](https://wordpress.org/plugins/pym-shortcode/) of `pym-shortcode`, which is what is most likely that this plugin has been installed under

- [ ] pick a text domain
- [ ] update all text-domain strings to that domain
- [ ] declare the text domain explicitly
- [ ] add note in PR template about new translatable strings matching the project one