Bash Compatibility: `+=` Operator

[eth-p]

conch-parser incorrectly parses the += operator.

In the context of array concatenation:

  arr+=(appended)
# ^^^^^ ^^^^^^^^ -- Subshell Command
# | -- Command

In the context of string concatenation:

  foo+="bar"
# ^^^^^^^^^^ -- Command: Concat([Simple(Literal("foo+=")), DoubleQuoted([Literal("bar")])])