Add support for slope customization (#599, #1165).

build-plans.toml

@@ -990,11 +990,6 @@ from = ["iosevka-aile"]
 release = true
 from = ["iosevka-etoile"]
 
-[collectConfig]
-distinguishWeights = true
-distinguishWidths = false
-distinguishSlope = false
-
 ###################################################################################################
 # Weight mappings (style => shape weight, menu weight, CSS weight)
 #   Shape weight : affects the shape of the glyphs
@@ -1048,13 +1043,24 @@ shape = 900
 menu  = 900
 css   = 900
 
-# slope mappings (style => CSS)
-# NOTE: This mapping does NOT affect the font's metadata, only affects
-#       the webfont CSS. Change `params/parameters.toml` instead.
-[slopes]
-upright = "normal"
-oblique = "oblique"
-italic = "italic"
+# slope mappings (style => slope angle, shape slope grade, menu slope, CSS slope)
+[slopes.upright]
+angle = 0
+shape = "upright"
+menu  = "upright"
+css   = "normal"
+
+[slopes.oblique]
+angle = 9.4
+shape = "oblique"
+menu  = "oblique"
+css   = "oblique"
+
+[slopes.italic]
+angle = 9.4
+shape = "italic"
+menu  = "italic"
+css   = "italic"
 
 # Width mappings (style => shape width, menu width, CSS stretch)
 #   Shape width   : affects the shape of the glyphs