refactor(ui5-flexible-column-layout): migrate to TypeScript

[hinzzx]

Migrated FlexibleColumnLayout component to TypeScript.

[nnaydenow]

Removing approve since there is open review comments

[ilhan007]

You may keep "0px" for the built-in layout configuration, but you need to ensure backward compatibility.
There are number of wrong states, caused by the change from 0 to "0px" (read the inline comments for more details).
In general, the code should work the same way with "0px" and 0. I suggest creating a method that checks against both the values and returns true if one of the conditions is true -returns v ==="0px" || v === 0. This will make the code working as before and letting us use only strings for the column layout.

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