A bit big, but practically it was a lot simpler to change over all
fillchars and all listchars at once, to not need to maintain two
parallel implementations.
This is mostly an internal refactor, but it also removes an arbitrary
limitation: that 'fillchars' and 'listchars' values can only be
single-codepoint characters. Now any character which fits into a single
screen cell can be used.
Problem: Many places in the code use `findoption()` to access an option using its name, even if the option index is available. This is very slow because it requires looping through the options array over and over.
Solution: Use option index instead of name wherever possible. Also introduce an `OptIndex` enum which contains the index for every option as enum constants, this eliminates the need to pass static option names as strings.
Problem:
Not all Lua code is checked by stylua. Automating code-style is an
important mechanism for reducing time spent on accidental
(non-essential) complexity.
Solution:
- Enable lintlua for `test/unit/` directory.
- TODO: only `test/functional/` remains unchecked.
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previous: 517f0cc634
