Fixes standalone mode & comment issues

[rart]

"Standalone mode" / Bundler-friendly

Using plugin-xml on web(pack) builds (in my case a react app), results in unusually long build times and compiler warnings. Looks like same issue as #28

Compiled with warnings.

./node_modules/prettier/index.js
Critical dependency: the request of a dependency is an expression

./node_modules/prettier/index.js
Critical dependency: the request of a dependency is an expression

./node_modules/prettier/index.js
Critical dependency: the request of a dependency is an expression

./node_modules/prettier/third-party.js
Critical dependency: the request of a dependency is an expression

./node_modules/prettier/parser-typescript.js
Module not found: Can't resolve '@microsoft/typescript-etw' in '/yada/yada/node_modules/prettier'

This PR switches print.js to require('prettier/standalone') instead of 'prettier' which works well in web builds and all (node) tests pass too.

Any objections to this approach? Alternatively there could be a standalone.js (same pattern that prettier itself follows) that does a whole different "build" of the plugin and somehow refactor print.js and plugin.js to minimize code duplication but seems unnecessary as this approach works fine.

Comment Issues

In addition to #13 and #14 I found another issue where a comment containing the < and > chars with a link inside gets utterly transformed.

Expected:

    <!--
    ~ Copyright (C) Some Corporation. All Rights Reserved.
    ~
    ~ Yada yada GNU General Public License
    ~ see <http://www.gnu.org/licenses/>.
    -->

Received:

    <http: //www.gnu.org/licenses="" />

I didn't realise about #30 until I had finished this but when I ran the changes on #30 with my XML updates - i.e. the ones on this PR - the tests didn't pass. Tests all pass with the changes on this PR as well as with the test XML on #30.

➜  prettier-plugin-xml git:(master) ✗ yarn test
yarn run v1.19.2
$ jest
 PASS  test/comment.test.js
 PASS  test/node.test.js
 PASS  test/attrs.test.js
 PASS  test/leaf.test.js
 PASS  test/arrays.test.js
 PASS  test/cdata.test.js
 PASS  test/location.test.js
 PASS  test/doctype.test.js

Test Suites: 8 passed, 8 total
Tests:       21 passed, 21 total
Snapshots:   0 total
Time:        1.256s
Ran all test suites.
✨  Done in 2.28s.

[rart]

@kddeisz wondering if you have any thoughts on this?

Also, once approved, what's the release cycle like?

[kddnewton]

@rart a bit slammed at work but I'll take a look this weekend. I can release immediately after merging.

[ghost]

@kddeisz This PR renders #30 moot. If this is accepted, #30 can be closed.

[ghost]

@kddeisz Can you please make a point of looking at this PR? I've had to remove this plugin from all of my (Maven) projects, as it's breaking my builds as it is currently.

[kddnewton]

Half of this is out of date now because of all the parser changes in #39. If you wouldn't mind retesting the standalone issues and reporting back to #28 then we can continue the discussion there