Introduce a function get excerpt length

[tremidkhar]

Currently, there is no way we can get the excerpt length whether the default or the filtered value.

This will be useful when developers in some way need to get the post excerpt length.

For now, developers can use something like the following to match the excerpt length with the of the site.

<?php
$excerpt_length = apply_filters( 'excerpt_length', 40 );

With the new function, developers don't have to remember to add the filter, they can just call the function and get the excerpt length that matches the rest of the site.

<?php
$excerpt_length = get_excerpt_length();

Trac ticket: https://core.trac.wordpress.org/ticket/56691