feat: allow language plugins to expose flags for module creation

[matt2e]

Each language plug in can expose flags for ftl new.

• Users can see these by using ftl new <language> --help
• FTL uses kong to set defaults, validate values, fill in envars, show help text, etc
  • This is done by adding kong.Flag items to kong's graph at start up based on the selected language.
• All flags are string values. It is up to each plugin to parse the values further (like go's replace flag being transformed into a map)

eg: this is appended at the bottom of ftl new go --help

Flags for Go modules
  -r, --replace=OLD=NEW,...    Replace a module import path with a
                               local path in the initialised FTL module
                               ($FTL_INIT_GO_REPLACE).

[matt2e]

@alecthomas is this a complete misuse of kong? 🤔 I figured this was nicer than kong's passthrough:"" as it makes it more discoverable

[alecthomas]

Very cool! I wondered how you were going to make that work.

[alecthomas]

@alecthomas is this a complete misuse of kong? 🤔 I figured this was nicer than kong's passthrough:"" as it makes it more discoverable

No way, this is super cool. I love how it's integrated seamlessly.