fix(language-service): Use ts.CompletionEntry for completions

[kyliau]

This is a prerequisite to fix a bug in template completions whereby
completion of the string ti for the variable title results in
tititle.

This is because the position where the completion is requested is used
to insert the completion text. This is incorrect. Instead, a
replacementSpan should be used to indicate the span of text that needs
to be replaced. Angular's own Completion interface is insufficient to
hold this information. Instead, we should just use ts.CompletionEntry.

Also added string enum for CompletionKind, which is similar to
ts.ScriptElementKind but contains more info about HTML entities.

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kyliau]

For Enum, the declaration itself should be CamelCase whereas the enum values should be CONSTANT_CASE.

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