fix(language-service): Do not show HTML elements and attrs for ext template

[kyliau]

This commit removes HTML elements and HTML attributes from the
completions list for external template. This is because these
completions should be handled by the native HTML extension, and not
Angular.

Once we setup TextMate grammar for inline templates, we could remove the
HTML completions completely.

Also added CompletionKind.HTML_ELEMENT, since there's already CompletionKind.HTML_ATTRIBUTE.

PR closes angular/vscode-ng-language-service#370

PR Checklist

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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
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• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

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• No

Other information

[kyliau]

caretaker: I'm a member of tools/language-service

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