Adds clear to levelup + updates tests

[carsonfarmer]

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: https://github.com/Level/levelup#clear
• If this PR brings the type definitions up to date with a new version of the JS library, update the version number in the header.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }. If for reason the any rule need to be disabled, disable it for that line using // tslint:disable-next-line [ruleName] and not for whole package so that the need for disabling can be reviewed.

[typescript-bot]

👋 Hi there! I’ve run some quick measurements against master and your PR. These metrics should help the humans reviewing this PR gauge whether it might negatively affect compile times or editor responsiveness for users who install these typings.

Let’s review the numbers, shall we?

These typings are for a package that doesn’t yet exist on master, so I don’t have anything to compare against yet! In the future, I’ll be able to compare PRs to levelup with its source on master.

Comparison details 📊

Batch compilation
Type count	9837
Assignability cache size	3188
Language service measurements
Samples taken	119
Identifiers in tests	119
getCompletionsAtPosition
Mean duration (ms)	299.3
Mean CV	11.5%
Worst duration (ms)	375.4
Worst identifier	on
getQuickInfoAtPosition
Mean duration (ms)	312.1
Mean CV	11.6%
Worst duration (ms)	376.1
Worst identifier	db
System information
Node version	v12.13.0
CPU count	2
CPU speed	2.294 GHz
CPU model	Intel(R) Xeon(R) CPU E5-2673 v4 @ 2.30GHz
CPU Architecture	x64
Memory	6.8 GiB
Platform	linux
Release	4.15.0-1061-azure

[typescript-bot]

@carsonfarmer Thank you for submitting this PR!

🔔 @MeirionHughes @danwbyrne - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

A definition owner has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!

[rbuckton]

Thanks for the contribution!

[typescript-bot]

I just published @types/levelup@4.3.0 to npm.

[carsonfarmer]

Woohoo, thanks for the quick reviews @rbuckton and @MeirionHughes!