Projectile params: consider adding alternative to "end"

"end" is a Lua keyword which makes it impossible to use neither of the usual `table.foo = 123` nor `table = { foo = 123 }` syntax.
https://github.com/beyond-all-reason/RecoilEngine/blob/f4d863a1e732638de5c019c720cd0575535881c8/rts/Lua/LuaSyncedCtrl.cpp#L442

Start at rts/Lua/LuaSyncedCtrl.cpp around line 442 and inspect how projectile parameters are exposed to Lua. Determine an alternative to the reserved `end` parameter name, then verify that Lua code can set the projectile value using the new option without the syntax error described in the issue.

Written by the indexing model from the issue text.