mv UNUSABLE_PASSWORD const to defines

disable code sniff for define check

move mail header constants to define file

Move defines to their own file

As described in
https://github.com/dwp-forge/columns/issues/5#issuecomment-638467603
sometime the Lexer constants have not been (auto)loaded when a syntax plugin
is in

Move defines to their own file

As described in
https://github.com/dwp-forge/columns/issues/5#issuecomment-638467603
sometime the Lexer constants have not been (auto)loaded when a syntax plugin
is invoked (I'm not sure why).

In general PSR2 discourages a mix of main code and function/class setup
with the call to define() being considered main code.

This patch moves these the define calls to a separate new file, solving
both of the above problems.

These are not all our defines. Instead I focused on the ones that are
ENUM-like.

In the future we should think about what defines can be replaced by
class constants and what other define() calls should be moved.

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