Make BeamPage.key required

[Goddchen]

Force BeamPage.key to be set in order for Navigator to be able to tell BeamPages apart.

[Goddchen]

@slovnicki Is it okay like this or should it be required LocalKey? key so that devs can set the key to null if they wish to? 🤔

[codecov]

Codecov Report

Merging #443 (a3d52e3) into master (a6bb5f3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #443   +/-   ##
=======================================
  Coverage   96.21%   96.21%           
=======================================
  Files          13       13           
  Lines         739      739           
=======================================
  Hits          711      711           
  Misses         28       28           

Impacted Files	Coverage Δ
package/lib/src/beam_page.dart	97.33% <ø> (ø)

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[slovnicki]

I now realize it must be pretty awkward for people to see this PR standing without attention like this.
This is because we talked about it privately, but yeah... Would be best I leave some comments here too.

Short story: This PR is waiting v2 because it will be a breaking change.

[slovnicki]

Well, @Goddchen, the time has come to merge this 😄

[Goddchen]

Omg, 2.0.0 incoming 🥳