Migration to Typescript

[posva]

Migrated the whole project to TS
Also added changes at vuejs/vue#10272

[codecov-io]

Codecov Report

Merging #302 into master will increase coverage by 0.05%.
The diff coverage is 99.66%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage   99.67%   99.73%   +0.05%     
==========================================
  Files           9       14       +5     
  Lines         308      371      +63     
  Branches       53       64      +11     
==========================================
+ Hits          307      370      +63     
  Misses          1        1

Impacted Files	Coverage Δ
packages/vuexfire/src/mutations-types.ts	100% <ø> (ø)
packages/vuexfire/src/firestore.ts	100% <100%> (ø)
...ackages/@posva/vuefire-core/src/firestore/index.ts	100% <100%> (ø)
packages/vuexfire/src/rtdb.ts	100% <100%> (ø)
packages/vuexfire/src/index.ts	100% <100%> (ø)
packages/@posva/vuefire-core/src/index.ts	100% <100%> (ø)
packages/vuefire/src/rtdb.ts	100% <100%> (ø)
packages/@posva/vuefire-core/src/rtdb/index.ts	100% <100%> (ø)
packages/vuexfire/src/mutations.ts	100% <100%> (ø)
packages/@posva/vuefire-core/src/shared.ts	100% <100%> (ø)
... and 16 more

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[posva]

Note to myself not to squash this one. I need to do a few local tests in projects to make sure everything still works the same