Add more assertions

[MarvinJWendt]

We should include more assertions:

• Add AssertSubset and AssertNotSubset #34
• Add AssertSameElements / AssertElementsMatch (irrelevant order) #33
• Add AssertDirExists and AssertNoDirExists #36
• Add AssertDirEmpty and AssertDirNotEmpty #37
• Add AssertFileExists and AssertNoFileExists #31
• Add AssertRegexp and AssertNotRegexp - asserts that regexp matches a string #28
• Add AssertIsDecreasing (on number slices) #29
• Add AssertIsIncreasing (on number slices) #30
• Add AssertLen #35
• Add AssertErrorIs and AssertNotErrorIs (wrapper of errors.Is) #32

[MarvinJWendt]

All done 🚀🥳