handled escaping characters at the start of lines.

Because of the quirks of markup it meant that it was losing the escaped character and that was causing all sorts of issues when it occurs at the start of a line.
Added a new specific rule before escaping occurs to catch this, not the best approach but it works for now.
Fixes #377

Tests/TestCases/Escaping.testplan

@@ -39,4 +39,9 @@ option: Here's a url: http://github.com/YarnSpinnerTool
 option: Here's some markup: hello
 option: Here's some escaped markup: [a]hello[/a]
 
-select: 1
+select: 1
+
+line: <<escaped command>>
+line: {0} escaped line with interpolation
+line: // escaped comment
+line: [a]escaped markup[/a]

Tests/TestCases/Escaping.yarn

@@ -39,7 +39,10 @@ Here's some escaped markup: \[a\]hello\[/a\]
 -> Here's some markup: [a]hello[/a]
 -> Here's some escaped markup: \[a\]hello\[/a\]
 
-// Commented out because this isn't actually allowed, but we're just
-// maintaining the pattern here:
+// testing the various escaped elements also work at the start of a statement:
+\<\<escaped command\>\>
+\{0\} escaped line with interpolation
+\/\/ escaped comment
+\[a]escaped markup\[/a]
 
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