Allow variables to have a leading digit

[flyingsilverfin]

Usage and product changes

We modify the behaviour of #310 which unified variables and labels to have the same valid identifier syntax, which eliminated the capability of variables to have a leading number. For example, the variable $0 was banned.

This PR reverts this specific behaviour, and enables usage of variables with leading digits:

match
$1_a isa entity;
get;

is made valid again.

Testing specified in typedb/typedb-behaviour#281

Implementation

We split the identifier regexes and grammar rules that validated labels and variables before, to be specific to Variable and Label identifiers.

[typedb-bot]

PR Review Checklist

Do not edit the content of this comment. The PR reviewer should simply update this comment by ticking each review item below, as they get completed.

---

Trivial Change

• This change is trivial and does not require a code or architecture review.

Code

• Packages, classes, and methods have a single domain of responsibility.
• Packages, classes, and methods are grouped into cohesive and consistent domain model.
• The code is canonical and the minimum required to achieve the goal.
• Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
• Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
• The code is algorithmically efficient and scalable for the whole application.

Architecture

• Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
• Any required build and release automations are updated and/or implemented.
• Any new components follows a consistent style with respect to the pre-existing codebase.
• The architecture intuitively reflects the application domain, and is easy to understand.
• The architecture has a well-defined hierarchy of encapsulated components.
• The architecture is extensible and scalable.

[flyingsilverfin]

TYPE_ prefix was a hangover from the past when "label" was called "type" in the grammar. Fixed here.

[flyingsilverfin]

Duplicated Label pattern here, so we don't end up with WB inside the scoped label? Is this necessary?

[dmitrii-ubskii]

We should probably have an IDENTIFIER_LABEL_ rule.

[dmitrii-ubskii]

Although the ANTLR grammar reuses LABEL_ instead. If we do that, IDENTIFIER_VAR_ might have to go as well. I'd rather we introduce both in ANTLR though.

[flyingsilverfin]

I just canceled them all, so we don't end up with confusion re. ~wb

[dmitrii-ubskii]

This can still be @{ LABEL_ ~ ":" ~ LABEL_ }