Enable `noUncheckedIndexedAccess` in DCR

[OllysCoding]

We should turn on noUncheckedIndexedAccess in our tsconfig.json: https://www.typescriptlang.org/tsconfig#noUncheckedIndexedAccess

/* Without 'noUncheckedIndexedAccess' */
const myArray: MyType = [];
const myVar = myArray[0];
// output: myVar: MyType

/* With 'noUncheckedIndexedAccess' */
const myArray: MyType = [];
const myVar = myArray[0];
// output: myVar: MyType | undefined

This is better for type safety & would make more sense in situations like: https://github.com/guardian/dotcom-rendering/pull/5694#discussion_r944357257

[OllysCoding]

This would require a lot of refactoring in DCR:

[mxdvl]

Some work has gone into catching errors, and we now stand at 249 errors only. Most of them come from arrays of unknown length in Front slices.

See also:

• #5829
• #5777
• #5804
• #5806