In Dejavu Sans Mono the old pointing triangles had a decent size,
but in Noto Mono and in Ubuntu Mono (fonts that handle combining
diacritics better) those triangles were massive, visually taking
up more than a single cell, overlapping the closing parenthesis
when they are displayed in the second column. Not nice.
The new pointing small triangles look good in the Noto and Ubuntu
fonts, but are real tiny in Dejavu. :| Oh well, one can't have
everything.
(Other triangles are not an option, because they don't degrade to
actual arrows when on a Linux console.)
Instead of quoting M- as 'M-', some translations use «M-» (Catalan),
(M-) (Croat), "M-" (Italian), „M-” (Romanian), or „М-“ (Serbian).
Those occurrences of M- nor its quotation marks should be colored.
Achieve this by excluding those quotation marks from the M- regex,
and then colorizing M-) and M-" only when at start of line (first
column of shortcuts) or when between parentheses (second column).
The "^_" has always been poorly legible (the underscore melting into
the bottom of the terminal) and difficult to type (Shift+Ctrl+minus).
The "^/" is both more readable and easier to type.
In terminal emulators, "^/" is shown, but on a Linux console "^-",
as ^/ does a backspace in the default console key mapping.
Also, allow using "^-" when rebinding ^_ in a nanorc file.
This addresses https://savannah.gnu.org/bugs/?57393.
Since ^@ is rebindable too and also a synonym of ^Space, it makes
more sense to allow the user to specify ^` instead, as it is an
unshifted keystroke on US keyboards.
Also, color ^` as valid in a nanorc file, and color ^` and ^@ in
the help viewer.