feat(editor): add Structured Text (IEC 61131-3) language support

[kaftanati]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactor
• Other, please describe:

Adds Structured Text (IEC 61131-3) to the editor language selector using the st language ID. Previously, Structured Text had no dedicated syntax highlighting and the legacy st mapping fell back to plain text.

The TextMate grammar highlights declarations, control flow, types, operators, PLC addresses, numeric and date/time literals, strings with dollar escapes, and nested block and line comments. The legacy mapping now preserves st.

Validations

• Follow our CONTRIBUTING guide
• 26 tests passed using the editor's TextMate engine: node node_modules/vitest/vitest.mjs run src/renderer/components/editor/grammars/__tests__/structuredText.test.ts.
• ESLint passed for the four changed files; git diff --check and commitlint passed.
• Electron UI was not launched; validation covers language registration and tokenization.

Literal syntax references: numeric literals, duration literals, string literals.