Add ABAP language support

[DreierF]

TanStack Highlight should ship an isolated language definition for ABAP, with the fence name abap.

We render a large number of ABAP code blocks, and they currently fall back to plaintext. Shiki supports ABAP out of the box, and highlight.js supports it through the third-party highlightjs-sap-abap plugin. A built-in definition would let us use the normal selective registration path instead of maintaining our own tokenization logic.

I have an implementation ready on the existing pattern helpers, with a small linear scanner for comments and literals. It handles:

• * comments in the first column versus multiplication, and " line comments
• '…' and `…` literals with doubled delimiters
• string templates |…{ expr }…|, where the { } holes are tokenized as code
• case-insensitive and hyphenated keywords (CLASS-METHODS, START-OF-SELECTION)
• field symbols <fs>, pragmas ##NEEDED, and class names before =>
• component selectors (-, ->, =>, ~), which keep keyword-like names plain, for example ls_entry-type

Deliberate limits: a variable named like a keyword (value, length, key) is highlighted as a keyword outside those positions. highlight.js behaves the same way.

Size: Core + ABAP is 3.64 KB gzip. The all-language entry grows from 17.68 KB to 18.94 KB gzip, mostly because of the keyword list. Core and the existing selective profiles are unchanged. Tests, a size profile, a benchmark profile, a fixture, docs, and a changeset are included, and pnpm run verify passes.