Twenty Twenty: use $theme_version variable for font stylesheet

[sabernhardt]

When [57311] added a font stylesheet,
wp_get_theme()->get( 'Version' )
identified the theme version within wp_enqueue_style().

1. twentytwenty_register_styles() already has a $theme_version variable for use with other stylesheets, and it could be reused.
2. twentytwenty_block_editor_styles() is the only function that includes wp_get_theme()->get( 'Version' ) without declaring the variable, and now the function uses it twice.
3. twentytwenty_register_scripts() and twentytwenty_customize_preview_init() have the variable but only use it once each.

[sabernhardt]

uses $theme_version variable in twentytwenty_register_styles() and adds the variable in twentytwenty_block_editor_styles()

[sabernhardt]

For 60779.diff​, I chose consistency. With that, all five of the enqueuing functions declare and use the $theme_version variable.

[SergeyBiryukov]

In 57842:

Twenty Twenty: Use the $theme_version variable for font stylesheet.

This aims to bring more consistency with the other wp_enqueue_style() calls in the theme's functions.

Follow-up to [57311].

Props sabernhardt.
Fixes #60779.