Add EXT_abort

[r-potter]

cc @gnl21

[arcady-lunarg]

Can this be merged? It's already implemented in glslang.

[rdb]

One grammar question: like GL_EXT_debug_printf, this extension doesn't actually define a syntactic path to allow a literal string in a function-call argument. The wording ("the first argument must be a literal string") suggests the string is a special argument rather than an expression; yet glslang parses strings as a primary-expression, so abortEXT(("foo")) is accepted by glslang even though nothing in this extension suggests that it should be.

So: what is actually intended here? I would like to know so that so that the grammar I have proposed in GL_EXT_spirv_intrinsics_string (#333) can be kept consistent with what GL_EXT_abort and GL_EXT_debug_printf do. Adding an explicit production to the extension text may be useful to remove any ambiguity.

Also, minor typo: "Last Modified Date" mentions "September 31, 2024", even though September has only 30 days.