Add array index support to expression grammar

[SteveL-MSFT]

PR Summary

Add array index syntax support to expressions.

• tree-sitter grammar updated which includes removing bracketInStringLiteral as that is no longer valid as array index results in a bracket within an expression
• as part of tree-sitter-grammar change, collapse member and index as accessors that follow a function
• added more tests to tree-sitter corpus which also needed to be moved to under a test folder to be discovered correctly now
• the DSC parser updated to use the new tree-sitter output and also added handling for errors following a valid statement

Fix #509