chore: Support feature flags in SSR

[just-boris]

Description

Make it possible to define feature flags in SSR

// remove dark header
global[Symbol.for('awsui-global-flags')] = { removeHighContrastHeader: true };

Related links, issue #, if available: n/a

How has this been tested?

Added an SSR test suite

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[just-boris]

Some unique symbol type madness here. Symbol.for(key) returns different symbols even if the key is the same: microsoft/TypeScript#35909

So I needed to add an import of awsuiGlobalFlagsSymbol. I also could not get it from const { ... } = globalFlags because it also would not be the same unique symbol.

But on the upside I managed to remove copies of these interfaces below while keeping everything type safe

[just-boris]

Decided to go with any here, because as ExtendedWindow all around this file were not adding much type safety anyway. But at least the code is more compact now

[connorlanigan]

We could use unknown for the parameter type and cast it to FlagsHolder inside the function. That way, we'll get type checks on the window?.[... line

[just-boris]

Could you show an an example of the readFlag(window: unknown) implementation you ideally want?

[just-boris]

Found a version from you: 7048f87#diff-5b9d78b3482fe99531e24bedf68759f075aeca010bbcae8797110683d36eb35cR17-R18

I get it now, updated the implementation

[connorlanigan]

For example:

function readFlag(object: unknown, flagName: keyof GlobalFlags) {
  if (!object) return undefined;

  const flagsHolder = object as FlagsHolder;

  return flagsHolder[awsuiGlobalFlagsSymbol]?.[flagName];
}

[just-boris]

Makes sense, window: any made the return type of readFlag to be any which is unexpected. Applied your suggestion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.84%. Comparing base (39e52a1) to head (119bb70).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2028   +/-   ##
=======================================
  Coverage   94.83%   94.84%           
=======================================
  Files         669      669           
  Lines       18113    18114    +1     
  Branches     6002     6003    +1     
=======================================
+ Hits        17178    17180    +2     
+ Misses        870      869    -1     
  Partials       65       65           

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[connorlanigan]

We could use unknown for the parameter type and cast it to FlagsHolder inside the function. That way, we'll get type checks on the window?.[... line

[connorlanigan]

typo: "there" should probably be "the"

[just-boris]

All these names are copy from the original tests. Updated them in both places