Add osmcarto theme

Uses the new flex configuration file currently discussed in
gravitystorm/openstreetmap-carto#4977

config/osmcarto.lua

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+-- ---------------------------------------------------------------------------
+--
+-- Example config for osmcarto theme
+--
+-- Configuration for the osm2pgsql Themepark framework
+--
+-- ---------------------------------------------------------------------------
+
+local themepark = require('themepark')
+
+-- ---------------------------------------------------------------------------
+
+themepark:add_topic('osmcarto/osmcarto')
+
+-- ---------------------------------------------------------------------------

themes/osmcarto/README.md

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+
+# Theme "osmcarto"
+
+This theme contains the definition for the OpenStreetMap Carto style from
+https://github.com/gravitystorm/openstreetmap-carto .
+
+There is only one topic called `osmcarto` available. Use like this:
+
+```{lua}
+themepark:add_topic('osmcarto/osmcarto')
+```
+
+Note that this contains a [copy of the
+config](topics/openstreetmap-carto-flex.lua) from the [openstreetmap-carto
+repository](https://github.com/gravitystorm/openstreetmap-carto). If that file
+is changed, it needs to be copied here.
+

themes/osmcarto/init.lua

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+-- ---------------------------------------------------------------------------
+--
+-- Theme: osmcarto
+--
+-- ---------------------------------------------------------------------------
+
+local theme = {}
+
+return theme
+
+-- ---------------------------------------------------------------------------