How to add Lightning Web Components as Custom Tabs in Salesforce

Form Winter '20 onwards you can add the Lightning Web Component as Custom Tab in Salesforce.

The <component>.js-meta.xml configuration file defines the metadata values for the component.
Add the lightning__Tab target to the component’s configuration file.

Ex:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="CustomSearchInLWC">
    <apiVersion>46.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__AppPage</target>
        <target>lightning__RecordPage</target>
         <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>

Path:
From Setup, enter Tabs in the Quick Find box, and then click on New button in Lightning Components Tabs

 Then Select your LWC component.

Result