Improve German morphology

[manuelaugustin]

The following parts of German morphology need to be improved:

Stemming

• When a word ends in -ln, the -n should be removed to form the stem, see https://trello.com/c/7UA6WwVW/3-when-a-word-ends-in-ln-the-n-should-be-removed-to-form-the-stem
• If an -er suffix is preceded by i (but not ei), it should not be removed, see https://trello.com/c/reG8065O/5-if-an-er-suffix-is-preceded-by-i-but-not-ei-it-should-not-be-removed
• Add exceptions for nouns that are treated as compounds of irregular nouns, even though they're not (e.g. "Reisepass" is a compound of "Reise" + "Pass", but "Kompass" isn't a compound based on "Pass")
• Don't stem verbs -ern from verbs ending in -rn (e.g., "schildern") https://trello.com/c/dD3oja2P/15-find-solution-for-stemming-issue-for-verbs-ending-in-rn

Verbs

• Add functionality for German participles Add functionality for German participles #2190
• Make sure that German words that have a vowel+e/i+vowel combination are stemmed correctly Make the German stemmer treat i and e like consonants if between vowels javascript#146
• Make sure that all irregular verbs are stemmed correctly Add irregular verb forms to German morphology exception check javascript#163
• Make sure that German verbs ending in -t are stemmed correctly German verbs ending in -t/-et aren't stemmed correctly javascript#189
• We can't correctly stem 3rd person singular verb forms ending in -t/-et for internal linking, see Create correct forms for german verbs ending in -t/-et javascript#199
• Stem past participles with separable + inseparable prefix (e.g., "wiederentdecken") https://trello.com/c/3iGq0lgt/24-german-past-participle-stemming
• And more to come (see Lingo Research Trello board)
• Add affixes for verbs ending in -l or -n, see https://trello.com/c/tp6AW8cj/13-review-some-rules-for-possible-verb-suffixes

Adjectives

Yoast/javascript#236

Parent of #2190

Requires https://github.com/Yoast/YoastSEO.js-premium-configuration/issues/14

[nataliashitova]

While acceptance testing the basic functionality introduced in #2153 I've noticed a bunch of cases that don't work. In consultation with @manuelaugustin we decided that they should be addressed separately.

sein

sein > Personal forms, imperatives, past participle not recognized.
I failed to receive a full paradigm for any of the input forms. I realize that this word would not likely to be used as a keyphrase, but for consistency and future Linguistic Tree usages it can be more than beneficial to have a nice sein.

haben

Verb forms are now generated using default (regular) rules. We need to add irregular forms, too.

verbs with separable prefixes

Verbs like einkaufen need special processing rules, which have not been implemented so far.

schenken and similar

Not all verb forms are recognized.
schenkende > Personal forms, imperatives, past participle not recognized.
geschenkt > No other forms are recognized.

[nataliashitova]

While working on Yoast/javascript#210 I've noticed that the function generateVerbExceptionFormsSpec fails to generate the form iss from any of the stems of the verb essen. We should investigate how local this problem is (just for this verb, only for verbs whose stems end with s, only for verbs of class 4, etc.).

[manuelaugustin]

All open points moved here: https://yoast.atlassian.net/browse/LIN-577