Closes pygments#2938.

Bicep is a domain-specific language for declarative deployment of
Azure resources. Adds a RegexLexer covering the syntax users actually
write day to day:

- Top-level declaration keywords (`param`, `var`, `resource`,
  `module`, `output`, `targetScope`, `type`, `func`,
  `metadata`, `extension`/`provider`, `import`, `extends`,
  `using`).
- Statement keywords: `if`, `else`, `for`, `in`.
- Built-in types: `string`, `int`, `bool`, `array`, `object`,
  the `secure*` variants, and the `sys` namespace marker.
- Constants: `true`, `false`, `null`.
- Curated function namespace -- the most-used `sys` / resource /
  string / file-loading functions get marked `Name.Builtin`. Bicep
  ships hundreds of functions; the curated list trades long-tail
  coverage for predictability so we don't pretend to know every
  function. Anything not on the list still highlights as Name.
- Decorators (`@description`, `@secure`, etc.).
- Single-quoted strings, triple-quoted multiline strings, and `${...}`
  interpolation that pops back into the root rule for nested
  expressions.
- Single-line `//` and block `/* */` comments.
- Resource API-version literals (e.g.
  `'Microsoft.Storage/storageAccounts@2023-01-01'`) work via the
  ordinary single-quoted string path -- no special case needed.
- Multi-character operators (`==`, `!=`, `<=`, `>=`, `&&`,
  `||`, `??`, `=>`, `..`, `?.`, `::`) tokenised before the
  single-char fallback so they don't mis-tokenise.

Files:

- `pygments/lexers/configs.py`: add `BicepLexer` class and append
  to `__all__`.
- `pygments/lexers/_mapping.py`: register `BicepLexer` between
  BibTeXLexer and BlitzBasicLexer (alphabetical), claiming
  `*.bicep` and `*.bicepparam` filenames.
- `tests/snippets/bicep/test_basic.txt`: golden snippet test
  covering decorators, declarations, types, the curated builtin
  function (`resourceGroup`), single-line + block comments,
  resource declaration with API-version literal, string
  interpolation, and an output binding.

Test plan:
  pytest tests/snippets/bicep/                -> 1 passed
  pytest tests/snippets/terraform/            -> 10 passed (untouched)
  pytest tests/test_basic_api.py              -> 2445 passed
  Manual visual on a real .bicep file looks correct (decorators,
  keywords, types, strings, interpolation all distinct)