fix(language-service): implement realpath to resolve symlinks

[kyliau]

The LanguageServiceAdapter must implement realpath in order to resolve
symlinks in node_modules.

Local libraries are often symlinked in node_modules by adding a local
dependency in package.json.

Fix angular/vscode-ng-language-service#1083

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• Bugfix
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What is the current behavior?

Issue Number: N/A

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Does this PR introduce a breaking change?

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