Make the micro copy for adding and installing modules consistent with the general plan for Drupal Core

Problem/Motivation

In #2888657: [meta] Less confusing and more consistent wording needed in module/theme add/install/update the there is the effort to unify and make the the wording for the add, install and update of modules and themes more consistent. Currently the button for module cards is using Download as the label as well as in the modal when you click the aforementioned download button you have the term download used for the first headline as well as within the text.

In the #3312289: Svelte UI for install controllers issue aside the terms Download and Enable are used as option labels.

For the record the issue was identified and initially discussed during #3312892: Drupal Usability Meeting 2022-10-07. The issue has a link to the recording of the meeting. The attendees were @AaronMcHale, @benjifisher, @narendraR, @rkoller, @shaal, @simohell, @srishtiiee, @Utkarsh_33, and @worldlinemine.

Proposed resolution

• Change the button label from Download to either Add or to be more sort of inline with the add button on /admin/modules to either + Add or + Add module
• Change the h3 title in the instructional modal from Download to Add
• Change the following text snippets within the first instructional section:
• The recommended way to download any Drupal module is with Composer. to The recommended way to add any Drupal module is with Composer.
• This will download the module to your codebase. to This will add the module to your codebase.
• If you cannot use Composer, you may download the module manually through your browser. to If you cannot use Composer, you may add the module manually through your browser.
• In the patch #3312289: Svelte UI for install controllers additionally change the option Download and Enable (experimental) to Add and Install (experimental)

Remaining tasks

• ✅ File an issue about this project
• ☐ Manual Testing
• ☐ Code Review
• ☐ Accessibility Review
• ☐ Automated tests needed/written?