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...

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/variable/expansion/test.out b/tests/variable/expansion/test.out
new file mode 100644
index 0000000..5056f04
--- /dev/null
+++ b/tests/variable/expansion/test.out
@@ -0,0 +1,6 @@
+FOO
+FOO
+FOO
+FOO
+foo
+BAR