Detect bad bullet point indentation in changelog entries

[gilles-peskine-arm]

Changelog entries are supposed to start with three spaces, an asterisk and a space. Continuation lines are supposed to be intended with five spaces. That is:

   * First line.
     Second line.

assemble_changelog.py does not enforce the indentation precisely because it's occasionally useful to have lines that are more indented. However, it should enforce more than it currently does.

For example, if a line begins with four spaces and an asterisk, it doesn't get rendered as expected in release notes (where we copy-paste the changelog): assuming the previous line isn't blank (it normally isn't: ), GitHub renders the asterisk as a text element rather than starting new bulleted list entry. This happens occasionally, e.g. “* Add mbedtls_pk_sign_ext() which allows generating RSA-PSS signatures…” and “* Extend the existing PSA_ALG_TLS12_PSK_TO_MS()” in https://github.com/Mbed-TLS/mbedtls/releases/tag/v3.2.0 .

I propose to enforce that every line starts with either " * " or " ".