Integrate ICU4X into Intl module (#2083)

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This Pull Request integrates an `ICU4X` data provider API in our codebase, to make use of the internationalization APIs that this crate provides.

It changes the following:

- Creates an API for pluggable icu data providers at `Context` creation, adding an `Icu` struct to store the provider (and some other internationalization tools) at runtime.
- Slightly changes locale related functions to preserve the `Locale` type and ensure correctness. (Will make some other changes related to this).
- Integrates the `sys_locale` crate to fetch the current default locale of an user instead of always returning `en-US`.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -34,7 +34,7 @@ Explain what you expected to happen, and what is happening instead.
 <!-- E.g.:
 Running this code, `a` should be set to `10` and printed, but `a` is instead set to `20`. The expected behaviour can be found in the [ECMAScript specification][spec].
 
-[spec]: https://www.ecma-international.org/ecma-262/10.0/index.html#sec-variable-statement-runtime-semantics-evaluation
+[spec]: https://tc39.es/ecma262/#sec-variable-statement-runtime-semantics-evaluation
 -->
 
 **Build environment (please complete the following information):**

.github/ISSUE_TEMPLATE/feature_request.md

@@ -16,7 +16,7 @@ Explain the ECMAScript feature that you'd like to see implemented.
 <!-- E.g.:
 I would like to see `switch` statement parsing and execution implemented. [ECMAScript specification][spec].
 
-[spec]: https://www.ecma-international.org/ecma-262/10.0/index.html#sec-switch-statement
+[spec]: https://tc39.es/ecma262/#sec-switch-statement
 -->
 
 **Example code**

.github/workflows/bors.yml

@@ -159,4 +159,4 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: doc
-          args: -v --document-private-items
+          args: -v --document-private-items --all-features

.github/workflows/master.yml

@@ -29,7 +29,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: doc
-          args: -v --document-private-items
+          args: -v --document-private-items --all-features
       - run: echo "<meta http-equiv=refresh content=0;url=boa_engine/index.html>" > target/doc/index.html
       - run: |
           if [ -d target/doc_upload ]; then rm -rf target/doc_upload; fi

.github/workflows/rust.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Run cargo-tarpaulin
         uses: actions-rs/[email protected]
         with:
-          args: --ignore-tests
+          args: --features intl --ignore-tests
       - name: Upload to codecov.io
         uses: codecov/codecov-action@v3
 
@@ -55,7 +55,7 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: test
-          args: -v
+          args: -v --features intl
 
   test_on_macos:
     name: Tests on MacOS
@@ -70,7 +70,7 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: test
-          args: -v
+          args: -v --features intl
 
   fmt:
     name: Rustfmt
@@ -163,4 +163,4 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: doc
-          args: -v --document-private-items
+          args: -v --document-private-items --all-features