WIP: Add US Symbolic layout for scientific literature

Added a new layout (US Symbolic) based on the default US keyboard map, edited for scientific literature. Added simple Greek letters (AltGr) to alphas in their common position as per Greek keyboard. Symbols commonly used in scientific literature are added (AltGr) everywhere else. TLDE and BKSL are AltGr-inverse, to facilitate placement of ESC over TLDE on ANSI 60% keyboards. LGST is a copy of TLDE, to facilitate placement of ESC over TLDE on ISO 60% keyboards.