TwentyNineteen: Add "Read More" Link Text When Using Post Excerpts

[alex27]

The excerpts do not have a "Read more" link, but display a [...] instead. While excerpts are not widely used in TwentyNineteen, they do appear in archive pages.

Originally suggested ​here

[sabernhardt]

We may want to create a twentynineteen_excerpt_more function, similar to the ​Twenty Sixteen filter.

Note: filters like that apparently are not compatible yet with the ​Latest Posts block (but the block editor issue needs fixing anyway for other themes' existing filters).

[metalandcoffee]

I created a patch for this. It's pretty much a direct copy of the excerpt_more filter function from the TwentySeventeen theme :)

[SergeyBiryukov]

In 49831:

Twenty Nineteen: Add "Continue reading" link for post excerpts.

This ensures that archive pages have a meaningful link text after the excerpts.

Props metalandcoffee, alex27, sabernhardt, mdrockwell, laurelfulford.
Fixes #46177.

[SergeyBiryukov]

In 49901:

Twenty Nineteen: Correct @since version for twentynineteen_excerpt_more().

Follow-up to [49831].

See #46177.