add missing key_indexer shorthand: Update default_config.lua

[rathod-sahaab]

This fixes bug #1118 where k is nil, in case of lua load from folder as it's not declared explicitly by the user.

[L3MON4D3]

Hey!
In case you're not aware, there is snip_env, a config-option which customizes the variables exposed in files loaded by the lua-loader (it's described here).
By setting it to {k = require("luasnip.nodes.key_indexer").new_key}, you can also get this variable :)

But since key-indexer is such a nice tool, you're probably right in wanting it in the default-env too, so TY for this PR :D

[L3MON4D3]

(Merged via cherry-pick in 04266a9)