How to best share code between projects in Quartus Prime Standard and Quartus Prime Pro ?

For VHDL: Quartus prime has some limited 2008 compatability. The IEEE_proposed library you mention is NOT part of the VHDL standard. That library name was used when David bishop created '93 compatible versions of the VHDL 2008 libraries, and made versions for each of the synth tools of the time (circa 2007 - so this code is 11 years old now!). You should be fine using it in Prime standard, you just have to include the code in your project and compile it like any other file. I was using it just fine 10 years ago.

As for what is supported:

In Pro: "Full" VHDL 2008 support

Prime Standard: https://www.intel.com/content/www/us/en/programmable/quartushelp/15.0/mergedProjects/hdl/vhdl/vhdl_list_2008_vhdl_support.htm

(basically, not a lot, and this is unlikely to change)

TBH, most 2008 features that are useful in synthesis can be done '93 code without too much difficulty. You have the '93 versions of the fixed_point libraries i mention above, and the supported features are mostly just ways to make the code a little less verbose. The truely new features come for testbenches and verification, which isnt supported in Quartus anyway. 3rd party simulators (like modelsim) have good VHDL 2008 support for these features.