fix(language-service): provide autocomplete results for incomplete element tags

[atscott]

(The first commit is included in #41054 and will get removed from this PR upon rebase)

Commit 1:
An opening tag < without any characters after it is interperted as a
text node (just a "less than" character) rather than the start of an
element in the template AST. This commit adjusts the autocomplete engine
to provide element autocompletions when the nearest character to the
left of the cursor is <.

Part of the fix for angular/vscode-ng-language-service#1140

Commit 2:
The current logic in the compiler is to bail when there are errors when
parsing a template into an HTML AST or when there are errors in the i18n
metadata. As a result, a template with these types of parse errors
will not have any information for the language service. This is because we
never attempt to convert the HTML AST to a template AST in these
scenarios, so there are no template AST nodes for the language service
to look at for information. In addition, this also means that the errors
are never displayed in the template to the user because there are no
nodes to map the error to.

This commit adds an option to the template parser to temporarily ignore
the html parse and i18n meta errors and always perform the template AST
conversion. At the end, the i18n and HTML parse errors are appended to
the returned errors list. While this seems risky, it at least provides
us with more information than we had before (which was 0) and it's only
done in the context of the language service, when the compiler is
configured to use poisoned data (HTML parse and i18n meta errors can be
interpreted as a "poisoned" template).

fixes angular/vscode-ng-language-service#1140

[atscott]

presubmit

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