Update declarations and assignments

[savq]

• Add local and global declarations.
• Split assignment_expression into assignment and compound_assignment_expression rules.
• Fix assignment allowing arbitrary expressions in the LHS.
• Rename bare_tuple_expression to bare_tuple.
• Add a separate rule for let bindings (these don't allow bare tuples).
• Update tests

---

Following Max's advise in https://github.com/tree-sitter/tree-sitter-julia/pull/6#issuecomment-663905837, I added an exception to parse :local as a quote expression. Note that currrently this is only a problem in an expression like

if x == :local
end

because this will be parsed as:

(if (== x :) (local end))

• Closes #11
• Closes #67