Add support for code completion inside text block templates #1038

[ktatavarthi]

What it does

Adds support for code completion within text blocks
This works in tandem with the JDT Core PR - eclipse-jdt/eclipse.jdt.core#1803.

This change adds the text block related jdt ui partition to the relevant code completion extensions.
Fixes #1038

How to test

1. Create a Java15 compliant or above project.
2. Create class C1 and main function in it.
3. Add the below code in the function
4. press Ctrl + Space, positioning the cursor in the text block after System.

String abc= STR."""
	\{System.}
	""";

The expected behavior is that the code completion suggestions should pop up at this stage.

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[jarthana]

I have tested the changes along with the relevant jdt.core changes and things are looking good.

[noopur2507]

Looks good. Thanks, Kalyan.