Highlight named funs called _, fix README and test record field types

[robertoaloi]

Follow-ups from syncing the grammar into the ELP VS Code extension, where a review turned up a few issues.

Highlight named funs called _

A named fun can be called _, but the name patterns required at least one character after a leading underscore. With a / later on the same line, the implicit fun rule took over, ended at the / and left end outside the fun:

m() -> fun _(A) -> A / 2 end.

_ is now accepted as a name in the named fun rule and in the clause name patterns of the fun expression rules.

Test where record field types start and end

No grammar change. A typed record field is scoped meta.type.erlang from its :: to the comma or brace that ends the field. The new tests show that a comma nested in the type (in a tuple, a fun type or a binary) doesn't end the region early, and that a default value, which Erlang writes before the ::, stays an expression:

-record(typed_fields, {a :: {integer(), fun((a, b) -> c)},
                       b :: <<_:8, _:_*8>>,
                       c = fun() -> ok end :: fun(() -> ok)}).

README fixes

• Fix a typo (ignode → ignore).
• Fix the path of the snapshot tests (./test/snap → ./tests/snap).
• Load the Markdown test grammar in the snapshot update command. Since Stop Markdown in doc attributes from leaking past the closing quotes #21 the snapshot tests run with it, but the documented command didn't load it, so running it as written rewrote the docstring and sigil snapshots with doc attributes scoped as generic directives.

Tests

npm test passes. The new tests for named funs called _ fail without the fix.