feat: support user customized snippets.

[ayamir]

This patch lazy loads user's customized snippets from lua/user/snips.
The usage is copy lua/snips to lua/user/snips/ first and create your snippets like the existing examples, i.e.:

cd ~/.config/nvim
mkdir -p ./lua/user/snips/
cp -vr ./snips/* ./lua/user/snips/
# remove existing snippets
cd ./lua/user/snips/snippets/
rm -rf *

Add your customized snippets for go.json like this:

touch go.json
nvim go.json

{
  "if err": {
    "prefix": "ie",
    "body": "if err != nil {\n\t$1\n}\n$2",
    "description": "Snippet for if err"
  }
}

Then you can use it:

Close #1385 @greggh

[charliie-dev]

LGTM!

[greggh]

I really don't know what to say. That was so fast. Thank you!