fix(window): set enter key as non-printable char

[kwonoj]

Mostly for discussion around #20630, #20726.
If this PR's accepted & merged, it may serve as temporal workaround only.

This PR changes behavior of window to consider ENTER key as non-printable, let document does not appends redundant carriage return when press enter on textinput field. But this PR instead omits keypress event on Enter like

while other browsers does emit

makes me feel proper fix would be something like #20726, revise flow to send over keyevent to document itself.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #20630 (github issue number if applicable).

• There are tests for these changes OR
• These changes do not require tests because _____

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @paulrouget: ports/servo/glutin_app/keyutils.rs

[paulrouget]

Having proper IME support would help solve these kind of issues.

[bors-servo]

☔ The latest upstream changes (presumably #21676) made this pull request unmergeable. Please resolve the merge conflicts.

[kwonoj]

Let me close this as upstream has changed.