Add a stable rule ID to the issue API

[nawforce]

Add a stable identifier to the issue API so consumers can refer to a specific kind of
diagnostic, rather than only its broad category.

Today Rule carries a name() and a priority(). name() is documented as "does need to be
unique but recommended it is", and in practice apex-ls supplies the DiagnosticCategory name —
Syntax, Error, Missing, Warning, Unused. Five values across every diagnostic apex-ls
produces, so there is nothing fine-grained enough to reference an individual issue type.

This blocks apex-dev-tools/apex-ls#322, which wants config-driven exclusion by issue ID
(e.g. missing-type). The identifier has to exist on the API before apex-ls can populate it.

Scope

Extend Rule with an id, in both cross-built sources:

• jvm/src/main/java/io/github/apexdevtools/api/Rule.java
• js/src/main/scala/io/github/apexdevtools/api/Rule.scala

Points to settle:

• Name and form. id() returning a String. Suggested convention is lowercase kebab-case,
  stable across releases once published, since it becomes part of users' config files.
• Relationship to name(). Whether name() stays as the human-readable label with id() as
  the machine key, or whether name() is eventually deprecated in favour of id().
• Default implementation. Rule is a published interface, so adding an abstract method is
  source- and binary-breaking for any external implementor. A default method on the Java side
  (returning name(), or an empty string) avoids that. Note the Scala.js side is a trait, so
  the two need to stay behaviourally aligned.

Issue.asString() currently formats as rule().name() + ": " + ... — worth deciding whether that
output changes, as apex-ls's CheckForIssues text/pmd output depends on it.

Why here and not in apex-ls

io.github.apexdevtools.api.{Issue, IssueLocation, Rule} is published from this repo, not apex-ls;
apex-ls consumes it transitively via outline-parser and shims onto it with DiagnosticRule.

The external analysis SPI that Rule was originally built for has been withdrawn — apex-ls-pmd is
deprecated and apex-ls#439 reduced getExternalAnalysis/setExternalAnalysis/
setExternalAnalysisMode to deprecated no-op stubs. In-tree, Rule is now consumed only by
apex-ls's CheckForIssues. It remains a published interface that third parties may implement,
though, so extending it is the safe move; removing it is not.

Downstream

Once released, apex-ls bumps outline-parser in build.sbt and assigns IDs at its ~106
diagnostic creation sites. Tracked in apex-dev-tools/apex-ls#322.