Lua: Support notifications with featured text

[XHawk87]

Is your feature request related to a problem? Please describe.
I want to be able to send notifications to players including tile, unit, and city links in them using lua.

Describe the solution you'd like
As documented in the Lua Reference Manual:

• Add link_text method to City type
• Add tile_link_text method to City type
• Add link_text method to Unit type
• Add tile_link_text method to Unit type
• Add link_text method to Tile type

Describe alternatives you've considered
It might technically be possible to manually duplicate the format for featured text in message strings but that would be ugly and messy.

Additional context
This was already been done in: https://github.com/freeciv/freeciv/blob/64f64a14ea220185058d8b57de2bf5c809169a18/common/scriptcore/api_game_methods.h#L220

[lmoureaux]

Can you pinpoint the commit on the fc side?

[XHawk87]

This looks like it: freeciv/freeciv@8bf3a07
Relating to: https://osdn.net/projects/freeciv/lists/archive/tickets/2021-December/002049.html

[lmoureaux]

Thanks! Doesn't seem difficult to port. Change function headers and ship it.