[Wildcard Variables] UNUSED_LOCAL_VARIABLE support

[pq]

As per the discussion in dart-lang/linter#4968, we want to start reporting UNUSED_LOCAL_VARIABLE for non wildcard underscore cases (e.g., __, ___, etc).

UPDATE: non-wildcard underscores will produce diagnostics but whether we want to start reporting on other identifiers (e.g., e or exception) is still an open question. (See proposal below.)

[pq]

Follow-up from dart-lang/linter#4974,

Proposal: report UNUSED_LOCAL_VARIABLE on all unused bound catch clause parameters

Specifically, this means we start flagging the following (which we don't today) with an UNUSED_LOCAL_VARIABLE diagnostic:

try {
} catch(e) { }

providing a quick-fix conversion to:

try {
} catch(_) { }

Since the idiom of catching (and ignoring) an exception named e or similar is well established, this will produce a bunch of diagnostics in some projects but with dart fix code can effortlessly be migrated.

/cc @dart-lang/language-team @dart-lang/analyzer-team