Add signature files for FSharpLspClient, Parser and UnionPatternMatch… #2026

Job	Run time
analyze	1m 45s
Build on windows-latest for repo global.json	12m 15s
Build on windows-latest for 6.0	13m 6s
Build on windows-latest for 7.0	13m 0s
Build on windows-latest for 8.0	13m 29s
Build on macos-latest for repo global.json	18m 55s
Build on macos-latest for 6.0	16m 40s
Build on macos-latest for 7.0	19m 52s
Build on macos-latest for 8.0	17m 48s
Build on ubuntu-latest for repo global.json	9m 38s
Build on ubuntu-latest for 6.0	9m 59s
Build on ubuntu-latest for 7.0	10m 10s
Build on ubuntu-latest for 8.0	10m 43s
	2h 47m 20s

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