move feature from %sqlrender -> %sqlcmd snippets

[edublancas]

we're going to deprecate %sqlrender, we should move the logic to %sqlcmd snippets

currently, this is how it works:

%%sql --save penguins
select * from penguins.csv limit 10

%%sql --save subset
select * from penguins where island is not null

cte = %sqlrender subset
print(cte)

prints:

WITH penguins AS (
select * from penguins.csv limit 10
)
select * from penguins where island is not null

Now, with the change, we should get the same when executing: %sqlcmd snippets subset

i.e.,

cte = %sqlcmd snippets subset
print(cte)

should print:

WITH penguins AS (
select * from penguins.csv limit 10
)
select * from penguins where island is not null

[bbeat2782]

Acceptance Criteria:

1. Make %sqlcmd snippets {name} gives the same output as %sqlrender {name}
2. Add FutureWarning to %sqlrender {name} that includes a link to %sqlcmd snippets documentation
3. Update %sqlcmd snippets documentation under API Reference tab to include explanation and behavior of %sqlcmd snippets {name}
4. Modify test functions for %sqlrender {name} to test %sqlcmd snippets {name}

[edublancas]

Add FutureWarning to %sqlrender {name} that includes a link to %sqlcmd snippets documentation

should also include the equivalent command. e.g., if user ran %sqlrender stuff, then the warning should tell the user to migrate to %sqlcmd snippets stuff

[bbeat2782]

Okay. I will include that in the error message that the FutureWarning shows.