Add support for evaluation context

For now it acts as just the value mode that can be enabled anywhere
variable expansion is supported, for example:

(foo=bar):

And the primary use currently is to enable/test quoted and indirect
variable expansion:

"foo bar" = FOO BAR
print $"foo bar"    # Invalid.
print $("foo bar")  # Yeah, baby.

foo = FOO
FOO = foo
print $($foo)

Not that you should do something like this...

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