Allow flymake diagnostic types to have shorter names

Allow flymake diagnostic types to have shorter names

Eglot, a third-party package which uses Flymake, has its own
diagnostic types such as 'eglot-error', 'eglot-warning', etc...  While
not being too long, they will not fit in the type column of the "list
all diagnostics" buffer.  This commit allows diagnostic types to have
user-defined names and also assigns names to the default categories.

* doc/misc/flymake.texi (Flymake error types): Describe
flymake-type-name prop.

* lisp/progmodes/flymake.el (flymake--diagnostics-buffer-entries):
Use type names.
(flymake-error, flymake-warning, flymake-note): Give these
diagnostic categories default type names.