Turns a template string into a flat list of tokens.

Tokens are one of:

• {:text, binary, line, col}
• {:tag, type, payload, line, col}

where type is one of :var, :unescaped, :section, :inverted, :close, :comment, :partial, :delimiter. The payload is:

• {:var | :unescaped, keys} — keys is the dotted name split on "."

• {:section | :inverted | :close, keys}

• {:partial, name} — the raw (unsplit) partial name
• {:comment, _} / {:delimiter, _} — content is irrelevant to output

Set-delimiter tags ({{=<% %>=}}) are applied during scanning so subsequent tags use the new delimiters.