Missing composite command for “i with macron”

[eg9]

Brief outline of the enhancement

It seems that some common accented letters aren't covered, in particular ī. Indeed

\=i

produces a macron over a dotted i. OT1 and T1 encoding should have a composed command for it.

The glyph is used in Latvian. While the UTF-8 direct input produces a correct output, it seems necessary to cover it for people who can't input the U+012B character directly.

Minimal example showing the current behaviour

\RequirePackage{latexbug}
\documentclass{article}

\begin{document}

\=i versus \={\i}
  
\end{document}

\RequirePackage{latexbug}
\documentclass{article}
\usepackage[T1]{fontenc}

\begin{document}

\=i versus \={\i}
  
\end{document}

Minimal example showing the desired new behaviour

\RequirePackage{latexbug}
\documentclass{article}

\DeclareTextCompositeCommand{\=}{OT1}{i}{\=\i}

\begin{document}

\=i versus \={\i}

\end{document}

\RequirePackage{latexbug}
\documentclass{article}
\usepackage[T1]{fontenc}

\DeclareTextCompositeCommand{\=}{T1}{i}{\=\i}

\begin{document}

\=i versus \={\i}

\end{document}

[Rimole]

See also #1075.