Adding check for escape character to resolve issue #6899

[victor-ludorum]

@lydia-duncan I have added check to resolve the issue Incorrect Syntax Highlighting For Escaped Character '"' #6899 by adding #string_escaped_char. As there is no check for the escape character provided in the chapel tmbundle .
Description of changes : The changes here done to correct the syntax highlighting in the Chapel syntax highlighter for escape characters . It lead to correct syntax highlighting for " ' ", " \ " ", , " \ a ", " \ b ", " \ f " , " \ e " , " \ n " , " \ r ", " \ t " , " \ v " and IPv6 addresses representation . Also previously in chapel tmbundle code at line 108
`

		<array>
			<dict>
				<key>include</key>
				<string>#string_escaped_char</string>
			</dict>
		</array>

`
but there is no description about #string_escaped_char in the code . The description of #string_escaped_char is also mentioned in this pull request to have correct syntax highlighting .

Kindly review the changes done .

[lydia-duncan]

Interesting thing - when playing with this change in TextMate as @LouisJenkinsCS suggested, I noticed that it works properly with single quoted strings for escaping both single and double, but that double quoted strings only allowed us to escape single quotes. i.e. these all work:

var x = 'blah\"morewords';
var y = 'blah\'morewords';
var z = "blah\'morewords";

but this didn't:

var fails = "blah\"morewords";

I suspect this means there is more to change.