Improve .npmignore of xterm-addon-search

You can see from https://unpkg.com/browse/xterm-addon-search@0.7.0/

that the xterm-addon-search npm packages contains unnecessary resources:

• fixtures
• out
• src (lib/xterm-addon-search.js.map already contains sources)

[jerch]

To solve the build target ambiguity, there is a new package.json entry exports since nodejs v12 (see here for an introduction). With this we can distribute ES6 builds together with default nodejs build (commonjs) in just one package. It is even possible to put an IIFE build as third goodie in the package so old style non ES6 script tags still work.

[mofux]

If we want browser builds to be distributed via npm, we should do a second package with main pointing to an ES6 module bundle of the addon. Those will not work in nodejs out-of-the-box, but the browser will be more than happy.

AFAIK we could use themodule field in package.json to point to the entry file of the ES6 Module bundle. The main should still point to the CJS version.