Implement TreeMenu, an interactive REPL folding-tree menu

[timholy]

This allows one to use a folding tree interactively. As described in the modified README (preview), on suitable Julia versions you can now display a folding tree, scroll through its options with the arrow keys, and collapse entire branches by toggling folding with the spacebar. Being able to implement this was my primary motivation for JuliaLang/julia#35915.

A couple of noteworthy points for any reviewers:

• the first commit just splits previous code into multiple source files. If you're reviewing, you can probably save yourself some trouble by only looking at the second commit.
• this PR bumps the package version to 1.0. I don't foresee any necessary API breakage, so other than this PR I am not sure what future development would justify moving to 1.0 if we don't do it now.
• there's some chance that the new writeoption (see the code) is of sufficiently general interest that I should put it in REPL.Terminals as writemaxchars or something. I am not sure how often people want to restrict text output to a single line, but I've found that when coupled with printstyled coloration, the obvious solution of just truncating a string has significant problems: you can leave the terminal in an unexpected mode, or even truncate in the middle of an ANSI terminal escape sequence. In contrast, this implementation seems quite robust so far.

[codecov-commenter]

Codecov Report

Merging #3 into master will increase coverage by 2.59%.
The diff coverage is 97.74%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
+ Coverage   95.16%   97.76%   +2.59%     
==========================================
  Files           1        4       +3     
  Lines          62      134      +72     
==========================================
+ Hits           59      131      +72     
  Misses          3        3              

Impacted Files	Coverage Δ
src/FoldingTrees.jl	100.00% <ø> (+4.83%)	⬆️
src/foldingtree.jl	95.00% <95.00%> (ø)
src/abstracttrees.jl	100.00% <100.00%> (ø)
src/treemenu.jl	100.00% <100.00%> (ø)

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