Remove requirement to use skipLibCheck

[vkarpov15]

What this PR does:

Currently astra-mongoose requires client projects set skipLibCheck: true to compile. That is a significant issue because skipLibCheck is all or nothing - in order to use astra-mongoose, client projects need to skip type checking for all node modules.

The major changes are:

1. Connections now use astraDb instead of db for storing the Astra DB wrapper. Mongoose assumes that db is a MongoDB compatible database wrapper; that means in order to avoid skipLibCheck we either need to have db be MongoDB compatible, or use a different property. I think having astraDb be separate makes more sense because we want to expose an Astra-specific wrapper. Although db -> astraDb is a backwards breaking change, I think it better aligns with the intended usage of this lib - astraDb is meant to be a more Astra-specific wrapper. If we try to make baseDb compatible, we need to implement a wide variety of methods like stats() etc. that throw an error (see here for reference implementation). The idea is that users can use Mongoose directly using this plugin, but also have astraDb as an escape hatch for accessing astra-db-ts directly, which lines up with how Mongoose is meant to be used.
2. options -> _options on collections. This is more correct because options() is a function on Mongoose collections.
3. Added TableUpdateFilter<DocType> to updateMany(), findOneAndUpdate() update type union. While not strictly correct currently because updateMany() and findOneAndUpdate() do not support tables, this satisfies the compiler.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CLA Signed: DataStax CLA

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[toptobes]

Although db -> astraDb is a backwards breaking change

You could try having something like

/** @deprecated use {@link astraDb} instead */
get db(): never {
  return this.astraDb as never;
}

to allow js-ers to keep using it at least while making it easier for ts users to migrate? Not sure if it's the best idea but just a suggestion to help w/ backwards compatability