Option to override environment variables in child processes

Today I have to restart Kakoune in order to set environment variables
in shell expansions globally. For example KAK_LSP_FORCE_PROJECT_ROOT.

When running ":git" commands, there is an ambiguity on whether to
use the $PWD or the buffer's worktree.  We use $PWD but it should be
possible to override this behavior.  An obvious way to do this is to
override Git environment variables:

	set-option -add global env GIT_WORK_TREE=/path/to/my/repo GIT_DIR=/path/to/my/repo.git

(another in-flight patch adds the obvious default to GIT_DIR so we don't need to set that).

---

There are some minor issues left
- If a user sets PATH, this will stomp the one we setenv()'d.
- Today both key and value require escaping for = and \. This is not
  intuitive. Since neither environment variables nor ui_options ever
  need a = in the key name, we can get rid of the escaping.

Alternative approach: The str-to-str-map can be somewhat clunky. We
could instead export any option named like env_FOO. Not yet sure
which approach is better.

Closes mawww#4482
That issues asks for a separate client-specific env as well but we
don't have a client scope so I'm not sure.

doc/pages/options.asciidoc

@@ -189,6 +189,9 @@ are exclusively available to built-in options.
     key is provided it removes that entry regardless of the associated
     value. +
 
+    Any `=` or `\` characters that in `key` and `value` must be escaped as
+    `\=` or `\\`.
+
     Only `str-to-str-map` options can be created with `declare-option`.
 
 == Builtin options
@@ -340,6 +343,9 @@ are exclusively available to built-in options.
 
     The default value is '%val{bufname} %val{cursor_line}:%val{cursor_char_column} {{context_info}} {{mode_info}} - %val{client}@[%val{session}]'
 
+*env* `str-to-str-map`::
+    a list of environment variable overrides to be passed to external processes.
+
 *ui_options* `str-to-str-map`::
     a list of `key=value` pairs that are forwarded to the user
     interface implementation. The NCurses UI supports the following options:

src/main.cc

@@ -574,6 +574,9 @@ void register_options()
     reg.declare_option<int, check_timeout>(
         "fs_check_timeout", "timeout, in milliseconds, between file system buffer modification checks",
         500);
+    reg.declare_option("env",
+                       "a list of environment variable overrides to be passed to external processes",
+                       HashMap<String, String, MemoryDomain::Options>{});
     reg.declare_option("ui_options",
                        "space separated list of <key>=<value> options that are "
                        "passed to and interpreted by the user interface\n"

src/shell_manager.cc

@@ -146,6 +146,8 @@ Vector<String> generate_env(StringView cmdline, const Context& context, GetValue
     static const Regex re(R"(\bkak_(quoted_)?(\w+)\b)");
 
     Vector<String> env;
+    for (const auto& [key, value] : context.options()["env"].get<HashMap<String, String, MemoryDomain::Options>>())
+        env.push_back(format("{}={}", key, value));
     for (auto&& match : RegexIterator{cmdline.begin(), cmdline.end(), re})
     {
         StringView name{match[2].first, match[2].second};