feat(help): generate :helptags using Treesitter

Problem:
Tags are manually parsed in C which is not flexible and prone to errors.
Extending the help system to allow for other formats (e.g. Markdown)
would require a large rewrite in the C core, while with Treesitter it
only needs a query update.

Solution:
Use the power of treesitter to extract the tags from helpfiles.

- build: set `$VIMRUNTIME` when generating helptags, like
  `cmake/Util.cmake` already does for other generators.
- fix(help): only accept tags delimited by whitespace. The old C parser
  only accepted a `*tag*` preceded by start-of-line or whitespace and
  followed by whitespace or end-of-line. The vimdoc parser also captures
  tags followed by other text, e.g. `*$XDG_STATE_HOME*/.../logs` in
  starting.txt, which caused an E154 duplicate tag error for docs that
  were previously fine.

test/functional/ex_cmds/help_spec.lua

@@ -1,11 +1,12 @@
 local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
 
-local describe, it, before_each, finally = t.describe, t.it, t.before_each, t.finally
+local describe, it, before_each, after_each, finally =
+  t.describe, t.it, t.before_each, t.after_each, t.finally
 local clear = n.clear
+local eval = n.eval
 local command = n.command
 local eq = t.eq
-local pcall_err = t.pcall_err
 local fn = n.fn
 local api = n.api
 local mkdir = t.mkdir
@@ -324,3 +325,83 @@ describe(':help', function()
     eq('*…*', api.nvim_get_current_line())
   end)
 end)
+
+describe(':helptags', function()
+  before_each(function()
+    for _, sfx in ipairs({ '', '2' }) do
+      fn.mkdir(('Xhelptags%s/doc'):format(sfx), 'p')
+      for _, tag in ipairs({ 'Xa', 'Xb' }) do
+        write_file(('Xhelptags%s/doc/%s%s.txt'):format(sfx, tag, sfx), ('*%s%s*'):format(tag, sfx))
+      end
+    end
+
+    clear()
+    command('set rtp+=Xhelptags,Xhelptags2')
+  end)
+
+  after_each(function()
+    rmdir('Xhelptags')
+    rmdir('Xhelptags2')
+  end)
+
+  it('requires an argument', function()
+    local msg = t.pcall_err(command, 'helptags')
+    eq(true, msg:find('E471') ~= nil)
+  end)
+
+  it('{dir}', function()
+    command('helptags Xhelptags/doc')
+
+    eq(eval("['Xa	Xa.txt	/*Xa*','Xb	Xb.txt	/*Xb*']"), eval("readfile('Xhelptags/doc/tags')"))
+
+    command('help Xa')
+    eq('*Xa*', api.nvim_get_current_line())
+  end)
+
+  it('ALL', function()
+    command('helptags ALL')
+
+    eq(eval("['Xa	Xa.txt	/*Xa*','Xb	Xb.txt	/*Xb*']"), eval("readfile('Xhelptags/doc/tags')"))
+    eq(eval("['Xa2	Xa2.txt	/*Xa2*','Xb2	Xb2.txt	/*Xb2*']"), eval("readfile('Xhelptags2/doc/tags')"))
+
+    command('help Xa2')
+    eq('*Xa2*', api.nvim_get_current_line())
+  end)
+
+  it('++t', function()
+    command('helptags ++t Xhelptags/doc')
+    eq('help-tags	tags	1', eval("readfile('Xhelptags/doc/tags')[-1]"))
+  end)
+
+  it('generates help-tag tag for VIMRUNTIME', function()
+    command('let $VIMRUNTIME="Xhelptags"')
+    command('helptags Xhelptags/doc')
+    eq('help-tags	tags	1', eval("readfile('Xhelptags/doc/tags')[-1]"))
+  end)
+
+  it('errors on duplicate tags', function()
+    -- duplicate tags in different files
+    write_file('Xhelptags/doc/Xd.txt', '*Xa*', nil, true)
+    local msg = t.pcall_err(command, 'helptags Xhelptags/doc')
+    eq(true, msg:find('E154') ~= nil)
+
+    -- tags file should still be generated
+    eq(1, eval("filereadable('Xhelptags/doc/tags')"))
+
+    os.remove('Xhelptags/doc/Xd.txt')
+
+    -- duplicate tags in same file
+    write_file('Xhelptags/doc/Xa.txt', '\n*Xa*', nil, true)
+
+    msg = t.pcall_err(command, 'helptags Xhelptags/doc')
+    eq(true, msg:find('E154') ~= nil)
+
+    eq(1, eval("filereadable('Xhelptags/doc/tags')"))
+  end)
+
+  it('with translated help files', function()
+    write_file('Xhelptags/doc/Xa.nlx', '*Xa*', nil, true)
+    command('helptags Xhelptags/doc')
+    eq(1, eval("filereadable('Xhelptags/doc/tags-nl')"))
+  end)
+end)