Add Snippets support and default snippets about dartvs

[sharwell]

You might benefit from separating this into two issues. For now, focus on creating the actual .snippet files and placing them in DanTup.DartVS.Vsix\Snippets. For example, you might create DanTup.DartVS.Vsix\Snippets\for.snippet for a for statement. I can help with implementing support for the snippets in completion lists.

[sharwell]

Assuming the for statement in Dart looks like it does in C, you could use this:

<?xml version="1.0" encoding="utf-8"?>
<CodeSnippets  xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
    <CodeSnippet Format="1.0.0">
        <Header>
            <Title>for</Title>
            <Shortcut>for</Shortcut>
            <Description>Code snippet for 'for' loop</Description>
            <Author>Sam Harwell</Author>
            <SnippetTypes>
                <SnippetType>Expansion</SnippetType>
                <SnippetType>SurroundsWith</SnippetType>
            </SnippetTypes>
        </Header>
        <Snippet>
            <Declarations>
                <Literal>
                    <ID>index</ID>
                    <Default>i</Default>
                    <ToolTip>Index</ToolTip>
                </Literal>
                <Literal>
                    <ID>max</ID>
                    <Default>length</Default>
                    <ToolTip>Max length</ToolTip>
                </Literal>
            </Declarations>
            <Code Language="Dart"><![CDATA[for ($index$ = 0; $index$ < $max$; $index$++)
{
  $selected$$end$
}]]>
            </Code>
        </Snippet>
    </CodeSnippet>
</CodeSnippets>

[sharwell]

Since this feature is easy to integrate into an existing completion provider, I've moved this milestone to match #5.