[Feature Request]: "subset" method for `qenv` object

[gogonzo]

Feature description

subset(names) method, should subset qenv object to keep only @env <- @env[names] and @code <- <@code for [names]>. Consider if it is worth exposing such function, or should we keep it as teal internal (where it is used). If we have a method to subset the code - then it makes sense to subset the whole object.

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[donyunardi]

Acceptance Criteria

• May need to decide on the name for the new method if not using subset
• Create a new method for qenv in teal.code
• Ensure to also able to subset join_keys in teal.data