feat: add new type definition for package mvdan-sh

[JounQin]

Please fill in this template.

• 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).
• The package does not already provide its own types, or cannot have its .d.ts files generated via --declaration
• If this is for an NPM package, match the name. If not, do not conflict with the name of an NPM package.
• Create it with dts-gen --dt, not by basing it on an existing project.
• Represents shape of module/library correctly
• tslint.json should be present and it shouldn't have any additional or disabling of rules. Just content as { "extends": "dtslint/dt.json" }. If for reason the some 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.
• tsconfig.json should have noImplicitAny, noImplicitThis, strictNullChecks, and strictFunctionTypes set to true.

[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 mvdan-sh with its source on master.

Comparison details 📊

Batch compilation
Type count	2188
Assignability cache size	118
Language service measurements
Samples taken	36
Identifiers in tests	36
getCompletionsAtPosition
Mean duration (ms)	86.1
Mean CV	25.6%
Worst duration (ms)	112.1
Worst identifier	KeepPadding
getQuickInfoAtPosition
Mean duration (ms)	83.6
Mean CV	25.6%
Worst duration (ms)	100.2
Worst identifier	syntax
System information
Node version	v12.16.1
CPU count	2
CPU speed	2.397 GHz
CPU model	Intel(R) Xeon(R) CPU E5-2673 v3 @ 2.40GHz
CPU Architecture	x64
Memory	6.8 GiB
Platform	linux
Release	4.15.0-1077-azure

[typescript-bot]

@JounQin Thank you for submitting this PR!

Because this is a new definition, a DefinitelyTyped maintainer will be reviewing this PR in the next few days once the Travis CI build passes.

In the meantime, if the build fails or a merge conflict occurs, I'll let you know. Have a nice day!

[typescript-bot]

After 5 days, no one has reviewed the PR 😞. A maintainer will be reviewing the PR in the next few days and will either merge it or request revisions. Thank you for your patience!