Spacing mark are not dead marks (non-spacing)

[moyogo]

Opening #46 (comment) as an issue:

The PR #46 reintroduces the glyph classes @deadMarks and @deadbelowMarks and the lookups DblMarks and CmbMarks using them to substitute spacing marks (modifier letters) by non-spacing marks (combining marks). These were removed in v1.2 but are now reused (except for /grave U+0060).

All spacing marks, no just /grave U+0060, must behave like spacing marks and remain spacing whether they follow a letter or not. Spacing marks characters should be used for spacing marks, non-spacing marks characters should be used for non-spacing marks. There should not be any substitution from spacing to non-spacing.

Dead keys are keyboard keys on mechanical typewriter that do not generate a character by themselves but modifies the character generated by the key struck immediately after.

Unicode 15.0, chapter 7, "Modifier Letters": Modifier letters are letters or symbols that are written adjacent to other letters and which modify their usage in some way. They are not formally combining marks and do no graphically combine with the base letter they modify

For example with "a´a˝aˆaˇa˘a˜a¯a¨aꞈaˍa¸a˛ ":

Current behaviour

Expected behaviour