feat(postfix): add templates

[kevinhwang91]

Postfix is extremely useful, but we should reduce its priority to avoid
missing the expected candidates during completing.

In the language server like gopls, which supports var to
assign variable and range to iterate collection, we should add
these templates for postfix in Lua as well.

[kevinhwang91]

The tests all failed because of the lower priority of postfix. I'm not sure you agree with my mind, waiting for your reply.

[sumneko]

If reduces its priority, it looks messy.

[sumneko]

The added templates looks nice.

[kevinhwang91]

If reduces its priority, it looks messy.

I have forgot that the trigger char for postfix has been changed to . by myself.
Have you tried the gopls with its postfix? The postfix snippet is triggered by . and its priority is lower than others like members of a struct.
In Lua language server, if I have typed table.in, and then accept the insert of the postfix template, the TextEdit will become table.insert(table, |) that is annoyed me a lot.

[sumneko]

I have reduce the priority by this commit: 8623701

You may open a separate PR for adding the templates.

[kevinhwang91]

force push now.

[sumneko]

Thank you!