Add ucode language

[janeblower]

Description

This adds detection and syntax highlighting for ucode, the JavaScript-like scripting language created for OpenWrt (interpreter by Jo-Philipp Wich). It is widely used across the OpenWrt ecosystem — openwrt/openwrt, openwrt/luci, and a large number of luci-app-* and related package repositories — for rpcd backends, LuCI controllers/dispatchers, and CLI utilities.

ucode's primary extension .uc is already registered to UnrealScript, so this is a shared-extension addition:

• both languages keep the .uc extension;
• UnrealScript already ships two .uc samples, and this PR adds three ucode samples;
• a content heuristic is added to heuristics.yml to disambiguate the two. ucode is matched on markers that never appear in UnrealScript (the #!/usr/bin/ucode shebang, 'use strict', ES import/export, require(/include(, let bindings, printf/sprintf), and UnrealScript on its own markers (class X extends Y, defaultproperties, var(...), #exec, simulated/exec/native function modifiers). let is used rather than const because UnrealScript also has const.
• a test (test_uc_by_heuristics) asserts both directions classify correctly.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension (ucode-isolating queries — a raw path:*.uc count is dominated by UnrealScript, so these use ucode-specific keywords):
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.uc+uci
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.uc+%22import%22+%22from%22
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.uc+ubus
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.uc+%22%23%21%2Fusr%2Fbin%2Fucode%22 (shebang — demonstrates ucode purity/no false positives)
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): openwrt/luci @ 2d05d310f53bac580808611dc4c70e95b22fee1b
      • samples/ucode/commands.uc — applications/luci-app-commands/ucode/controller/commands.uc
      • samples/ucode/ddns.uc — applications/luci-app-ddns/root/usr/share/rpcd/ucode/ddns.uc
      • samples/ucode/http.uc — modules/luci-base/ucode/http.uc
    • Sample license(s): Apache-2.0
  • I have included a syntax highlighting grammar: https://github.com/NoahBPeterson/ucode-lsp (scope source.ucode, MIT)
  • I have added a color
    • Hex value: #00b8d4
    • Rationale: ucode has no official brand color; selected a cyan that is visually distinct from nearby language colors. Happy to adjust.
  • I have updated the heuristics to distinguish my language from others using the same extension.