[REQ] Enforce spaces as indent keywords for indented blocks

[mhatzl]

All indented blocks must only allow spaces as indent keywords.
e.g. Block Quote must not allow ">" as indent keywords besides spaces.

Valid indenting:

> Block Quote
  That has valid indenting

Invalid indenting:

> Block Quote
> This is invalid.

Why?

It makes parsing much easier, because prefix matching can be changed to number of spaces to skip.
Rendering back to Unimarkup also gets easier, because the indent keyword will always be a space.

Affects

• Heading
• Line Block
• Block Quote

Lists already only allowed spaces for indenting, so they are not affected.

For headings, it should also be allowed to specify another heading of any level directly before or after.
Compilers might still issue warnings if levels are not increasing by exactly one level.

# First heading lvl 1
# Second heading lvl 1 (warning might be set)

## First heading lvl 2
# Third heading lvl 1 (warning might be set)