Add explicit types

[Tyriar]

Fixes #898
Fixes #901
Fixes #903

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This change brings in manual typings, previous the automatically generated typings via the declaration property in tsconfig.json contained everything which is not what we want to expose to consumers. After this goes in, typings/xterm.d.ts contains the complete API (excluding addons currently).

There are also simple tests in fixtures/typings-test/ that ensure that TS code compiled against the typing files compiles as expected. I found a bug while writing this which is always a good sign that it's useful 😄

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TODO list:

• Only allow the actual event types in on ('a' | 'b')
• Provide types in on event listeners https://xtermjs.org/docs/api/terminal/#events
• Only allow the actual addon types in loadAddon ('a' | 'b')
• Ensure comments work for functions with overloads (getOption/setOption)
• Add a comment for each function with an overload
• Remove general function declarations when there are overloads?
• Consider adding addon interfaces to the .d.ts (maybe later?)
• Add the link matcher API with experimental notice
• Can we test this? We want CI to reject things like Terminal.scrollDisp getting removed or renamed
• Should we remove the overloads in getOption, etc. from src/ so they're not duplicated?
• Add tests for bellSound/bellStyle added in If bell() gets called emit bell event and add default bell sound #878
• Clean up and finalize valid options Remove dead options from DEFAULT_OPTIONS #897

[Tyriar]

Neat 😄

[coveralls]

Coverage decreased (-0.5%) to 71.335% when pulling 3ba3638 on Tyriar:898_explicit_typings into ae0d5e4 on sourcelair:v3.

[coveralls]

Coverage remained the same at 71.864% when pulling 92ac427 on Tyriar:898_explicit_typings into ae0d5e4 on sourcelair:v3.

[Tyriar]

[coveralls]

Coverage decreased (-0.5%) to 71.335% when pulling e4e2fbf on Tyriar:898_explicit_typings into ae0d5e4 on sourcelair:v3.

[coveralls]

Coverage increased (+0.04%) to 71.899% when pulling 6e2ca0a on Tyriar:898_explicit_typings into ae0d5e4 on sourcelair:v3.

[Tyriar]

Ready for review, updated top comment to explain the change more.

[coveralls]

Coverage increased (+0.05%) to 71.915% when pulling 34f5b2c on Tyriar:898_explicit_typings into ae0d5e4 on sourcelair:v3.

[coveralls]

Coverage increased (+0.04%) to 71.838% when pulling 34e1ee2 on Tyriar:898_explicit_typings into 9ab17de on sourcelair:v3.

[coveralls]

Coverage increased (+0.04%) to 71.838% when pulling f520e27 on Tyriar:898_explicit_typings into 9ab17de on sourcelair:v3.

[coveralls]

Coverage increased (+0.04%) to 71.838% when pulling f520e27 on Tyriar:898_explicit_typings into 9ab17de on sourcelair:v3.

[coveralls]

Coverage increased (+0.06%) to 71.851% when pulling 8fdedd4 on Tyriar:898_explicit_typings into 9ab17de on sourcelair:v3.

[coveralls]

Coverage increased (+0.08%) to 71.877% when pulling c984013 on Tyriar:898_explicit_typings into 9ab17de on sourcelair:v3.

[coveralls]

Coverage increased (+0.08%) to 71.851% when pulling a0d8e3c on Tyriar:898_explicit_typings into f251047 on sourcelair:v3.

[parisk]

LGTM 👍. Two questions:

1. Can we be a bit more lenient with the bellStyle option in order to allow null and convert it internally to none?
2. Should we ditch the geometry option (Deprecate geometry option in favor of rows and cols #841)?

[Tyriar]

@parisk

1. 👍
2. I was starting to go through the breaking changes but thought maybe I should stop as the PR is already huge.

[Tyriar]

I allowed null for cursorStyle as well since it's similar to bellStyle.

[coveralls]

Coverage increased (+0.004%) to 71.773% when pulling eda9443 on Tyriar:898_explicit_typings into f251047 on sourcelair:v3.