Existing elements of an existing dependency got broken when inserting artifactId from CA

[vrubezhny]

Steps to reproduce:

• Consider editing Maven Project like the following:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <artifactId>foo-child</artifactId>
  <packaging>pom</packaging>
  <name>FOO Child</name>
  <dependencies>
	<dependency>
		<artifactId>org.ecl|</artifactId>
	</dependency>
  </dependencies>
</project>

• Place the cursor in text instead of "|" character and press Ctrl-Space to invoke the content assistant
• Continue typing characters, for example ipse.lem in order to make org.eclipse.lemminx artifact to be the first selected proposal, then insert the selected proposal
• See the proposal inserted breaking the existing code:

	<dependency>
		<groupId>org.eclipse.lemminx</groupId>
		<artifactId>org.eclipse.lemminx</art
		<version>0.19.1</version>ifactId>
	</dependency>

[vrubezhny]

The main fix is made in LSP4E: eclipse/lsp4e#89
The PR #285 contains additional fix for ` element in CA
The PR #286 contains new JUnit test cases made for the issue (depends on lemminx project PR - eclipse/lemminx#1186)