Make quote marks consistent in error/warning messages (#1791)

- "Double quotes" for strings (filenames, section names, CLI option arguments, etc)
- 'Single quotes' for characters and CLI option flags
- `Backticks` for keywords and identifiers (symbol names, charmap names, etc)

CLI option flags also have their leading dashes

test/gfx/input_tileset_deduped.err

@@ -1,2 +1,2 @@
-error: The input tileset's tile #7 was deduplicated; please check that your deduplication flags (`-u`, `-m`) are consistent with what was used to generate the input tileset
+error: The input tileset's tile #7 was deduplicated; please check that your deduplication flags ('-u', '-m') are consistent with what was used to generate the input tileset
 Conversion aborted after 1 error

test/gfx/input_tileset_extra.err

@@ -1,4 +1,4 @@
-error: Tile at (0, 0) is not within the input tileset, and `-o` was not given!
-error: Tile at (0, 8) is not within the input tileset, and `-o` was not given!
-error: Tile at (8, 8) is not within the input tileset, and `-o` was not given!
+error: Tile at (0, 0) is not within the input tileset, and '-o' was not given!
+error: Tile at (0, 8) is not within the input tileset, and '-o' was not given!
+error: Tile at (8, 8) is not within the input tileset, and '-o' was not given!
 Conversion aborted after 3 errors

test/gfx/none_comma_omission.err

@@ -1,4 +1,4 @@
 error: Unexpected character, expected ',', ';', or end of argument
-In inline palette spec: #000,#none#fff
-                                  ^
+In inline palette spec: "#000,#none#fff"
+                                   ^
 Conversion aborted after 1 error