Adds a virtual node to `DefaultValuePattern` to end the
`ExprRegionForPattern` before checking the expression
for the default value.
When checking the default value expression, the context
was still configured to interpret expressions as patterns,
which caused some corruption of state with tuple-pattern
subpatterns.
Corrects an assertion failure I found while working on
feedback from #7665.
Adds parsing support only for default value expressions in pattern
lists, including leaving the default value unspecified with an
underscore `_`.
Per #7521.
It's hard to track 2-space indent levels across large vertical gaps, and
it can be hard to suppress the instinct to read the dump as if it were
preorder. This change introduces a new dump mode that addresses both
problems by using box-drawing characters to explicitly represent the
parent-child edges of the tree. This new mode is the default, but the
old behavior remains available with
`--parse-dump-format=yaml-postorder`.
Lucile Rose Nihlen
Geoff Romer
oli-ej