Add new actions before and after loading the located template

[Tabrisrp]

This patch adds new actions before and after loading the located template in locate_template().

The template filename is passed as an additional parameter to the action, as that can be useful for callbacks on these hooks.

The hooks name and the @since value are suggestions, I couldn't find if there is a standard for hooks names in core, as there is some prefixed with wp_, and others that are not.

Trac ticket: https://core.trac.wordpress.org/ticket/54541

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[peterwilsoncc]

I've added a couple of minor notes inline, nothing to big.

I think @johnbillion wanted to review this too so maybe wait until you hear from John before taking any action. John's been around longer than me so may have additional advice.

[martinkrcho]

@Tabrisrp , would you be able to apply the changes suggested by @peterwilsoncc? Then we could get @johnbillion to review and I would be more than happy to contribute with some unit tests.

[Tabrisrp]

@martinkrcho Done!

[peterwilsoncc]

Thank you. This code looks good to me.

I think John has limited availability at the moment, so let's give him a week or two to add any thougths to this ticket.

[johnbillion]

This looks good but it would be good to pass some additional contextual parameters to the actions. I think the $template_names, $require_once, and $args parameters are all potentially useful to these hooks, so let's pass them in please.

[johnbillion]

Committed in https://core.trac.wordpress.org/changeset/53560