[UI v2] Refactor parts of variables components

[desertaxle]

Now that the core functionality is in place, I went through what's been created so for to refactor and polish. The code of this refactor is to reduce prop drilling, increase readbility and increase reusability of existing functionality.

Here are some of the highlights in no particular order:

• The existing queries for variables and variables counts have been consolidated into a single useVariables hook.
• All query and mutation hooks have been collocated and query_key creation has been consolidated to prevent typo-based errors.
• The pagination, filtering, and sorting state management has been consolidated into their own hooks.
• Variable dialog state management has been encapsulated in a hook.
• The search parameter validation has been updated to gracefully handle search parameters that don't conform to the expected typing.

Related to #15512

[aaazzam]

if the tag name has one character, it'll error out with an "undefined" error

[aaazzam]

filtering by variable name doesn't require pressing return, filtering by tag does. (I get why, but it's a little annoying -- not blocking)

[aaazzam]

[aaazzam]

[aaazzam]

[aaazzam]

the "X" in the sonner feels like it should be vertically aligned