Fix BG localization grammatical errors

[David-Mavrodiev]

Datepicker: Fix multiple grammar mistakes in BG localization file

There is a grammatical rule in Bulgarian language when word is shortened - the last letter must be always consonant. However, for months and days there are official abbreviations, as follows: Days -> "Нед.", "Пон.", "Втор.", "Ср.", "Четв.", "Пет.", "Съб." Months -> "Ян.", "Февр.", "Мар", "Апр.", "Май", "Юни", "Юли", "Авг.", "Септ.", "Окт.", "Ноем.", "Дек."

Also year should have suffix "г." for example "2020 г." "closeText", "prevText", "nextText", currentText must start with capital letter.

For reference

• https://ibl.bas.bg/ezikovi_spravki/ima-li-ofitsialno-pravilo-za-sakrateni-imena-na-mesetsite-na-balgarski/

[jsf-clabot]

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[fnagel]

Thanks for your contribution!

Before merging this PR we'll need some review of the localization file, see https://jqueryui.pbworks.com/w/page/139157592/Adding%20translations

Please fix the CGL issue and change commit message to something like "Datepicker: Fix Bulgarian localization grammatical errors"