Revert "Make TerminalUI::get_next_key() helpers static"

On macOS, backspace reportedly no longer works after <c-z> and fg.
The value of m_original_termios.c_cc[VERASE] seems to be wrong
in a static lambda that captures a singleton "this".
Not sure what's the problem. I thought that it is guaranteed that
"static auto convert = [this]() { ... }" is initialized lazily,
hence it should capture the correct address. Maybe the address
changes somehow or it's UB / a compiler bug.

This reverts commit ad36585

Closes #5155

src/terminal_ui.cc

@@ -702,7 +702,7 @@ Optional<Key> TerminalUI::get_next_key()
 
     static constexpr auto control = [](char c) { return c & 037; };
 
-    static auto convert = [this](Codepoint c) -> Codepoint {
+    auto convert = [this](Codepoint c) -> Codepoint {
         if (c == control('m') or c == control('j'))
             return Key::Return;
         if (c == control('i'))
@@ -717,7 +717,7 @@ Optional<Key> TerminalUI::get_next_key()
             return Key::Escape;
         return c;
     };
-    static auto parse_key = [](unsigned char c) -> Key {
+    auto parse_key = [&convert](unsigned char c) -> Key {
         if (Codepoint cp = convert(c); cp > 255)
             return Key{cp};
         // Special case: you can type NUL with Ctrl-2 or Ctrl-Shift-2 or
@@ -756,7 +756,7 @@ Optional<Key> TerminalUI::get_next_key()
         return mod;
     };
 
-    auto parse_csi = [this]() -> Optional<Key> {
+    auto parse_csi = [this, &convert]() -> Optional<Key> {
         auto next_char = [] { return get_char().value_or((unsigned char)0xff); };
         int params[16][4] = {};
         auto c = next_char();