Add localization system

[humanapp]

Add build-time localization runtime

Adds the localization foundation for the microbit-apps stack: English source
strings are both the catalog keys and the runtime fallback, and exactly one
language ships per build image.

What changed

• loc.ts (new): the _loc namespace holds two seams assigned at startup
  by a consuming app's generated file: table (source string -> translation)
  and defaultFont. Public API:
  • ui.loc(s): table lookup with fallback to the source string; identity
    when no table is assigned, so English builds pay no per-string cost.
  • ui.locc(context, s): context-disambiguated lookup under the catalog key
    context#string, falling back to the plain-string translation, then to
    the source string. For the rare case where one English word needs
    different translations in different places.
  • ui.locf(s, args): positional {0}-style interpolation applied after
    translation, so translations can reorder placeholders.
  • ui.locFont(): the per-language default font, or bitmaps.font8.
• pxt.json: loc.ts registered immediately after ns.ts so the
  runtime initializes before any file that calls it.
• test.ts: runLocTest() covering the identity path, table hit and
  miss, the full locc fallback chain, reordered-placeholder interpolation,
  and locFont default/assigned behavior, with state resets between groups.
• locales/en.json + scripts/locstrings.mjs: the source-string catalog
  (empty; the library has no user-visible strings of its own) and the
  extractor that regenerates it from ui.loc(...) call sites. Arguments must
  be string literals; anything else is reported loudly. locstrings-ignore
  on a call's line suppresses deliberate dynamic pass-throughs, and the
  file implementing the loc API itself is skipped.
• package.json: npm run loc:strings.

Behavior

No behavior change for existing consumers: nothing in ui-core calls the new
API, and with no table assigned every function is identity/fallback.
UiAssetResolver.getText is unchanged; the two mechanisms compose (a
resolver implementation may return ui.loc(...) results).

Verification

• mkc build green, including test files.
• Test assertions executed in the simulator.

[Copilot]

Pull request overview

Adds a build-time localization runtime to ui-core, establishing a minimal translation mechanism (source-string keys with English fallback) plus an extraction script to build the English catalog from ui.loc* call sites. This sets up the localization foundation for consuming microbit-apps packages while keeping English builds fast by omitting the translation table.

Changes:

• Introduces ui.loc, ui.locc, ui.locf, and ui.locFont backed by an app-assigned _loc.table and _loc.defaultFont.
• Adds a catalog extraction script (scripts/locstrings.mjs) and an empty locales/en.json, plus an npm script to regenerate the catalog.
• Adds a localization test harness and wires loc.ts into pxt.json file ordering.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
loc.ts	Adds the localization runtime API (loc, locc, locf, locFont) and _loc seams for app-provided table/font.
test.ts	Adds runLocTest() coverage for identity behavior, table hits/misses, locc fallback chain, interpolation, and default/custom font.
scripts/locstrings.mjs	Adds a source scanner to regenerate locales/en.json by extracting literal strings from ui.loc* call sites.
locales/en.json	Adds the generated English-source catalog (currently empty).
package.json	Adds npm run loc:strings to run the extractor.
pxt.json	Registers loc.ts early in the build file list and rearranges existing keys.

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