Added filter for updating text when not enrolled in any course

[actuallyakash]

Description

Added a filter for updating text when the user is not enrolled in any course.

Fixes #2365

How has this been tested?

Manually.

Screenshots

A user can use the below filter to update text:

function custom_not_enrolled_text( $text ) {
    return __( 'Oops! Looks like you haven\'t signed up for any courses yet.', 'lifterlms' );
}
add_filter( 'lifterlms_not_enrolled_text', 'custom_not_enrolled_text' );

Types of changes

Bug Fix.

Checklist:

• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.

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[seothemes]

@actuallyakash looks good, I would just add a docblock for the new filter (see comment)

Also need to run npm run dev changelog add -- -i to add the changelog entry

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[eri-trabiccolo]

@actuallyakash
once fixed these minor things I think we're good to go <3

[eri-trabiccolo]

Thanks @actuallyakash